Exhibit 99.4

Horizon Pharma plc

UNAUDITED PRO FORMA COMBINED FINANCIAL INFORMATION

Horizon Pharma plc Unaudited Pro Forma Combined Financial Statements

On March 18, 2014, Horizon Pharma, Inc. (“HPI”), Vidara Therapeutics Holdings LLC, a Delaware limited liability company (“Vidara Holdings”), Vidara Therapeutics International Ltd, a private limited company formed under the laws of Ireland (“Vidara”), Hamilton Holdings (USA) Inc., a Delaware corporation (“US HoldCo”), Hamilton Sub Inc., a Delaware corporation and wholly owned subsidiary of HoldCo (“Merger Sub”), entered into a Transaction Agreement and Plan of Merger (the “Merger Agreement”). The following unaudited pro forma combined financial statements give effect to (i) pursuant to the terms and conditions of the Merger Agreement, the merger of Hamilton Merger Sub Inc. with and into HPI, with HPI continuing as the surviving corporation and as a wholly-owned, indirect subsidiary of Vidara (the “Merger”) being accounted for as a reverse acquisition, with HPI being deemed the acquiring company for accounting purposes, (ii) the incurrence of US $300 million of debt by HPI, and (iii) the acquisition of VIMOVO^® rights (“AZ VIMOVO” or “ VIMOVO Acquisition”) by HPI including the related financing, which was completed on November 18, 2013 (collectively, the “Pro Forma Transactions”), on the financial position and results of operations of Horizon Pharma plc (“New Horizon”). The unaudited pro forma combined balance sheet as of June 30, 2014 gives effect to the Pro Forma Transactions described in clauses (i) and (ii) as if such transactions had occurred on June 30, 2014. The unaudited pro forma combined statements of operations for the year ended December 31, 2013 and the six months ended June 30, 2014 give effect to the Pro Forma Transactions as if they occurred on January 1, 2013.

Because the security holders of HPI owned approximately 74% of the fully-diluted capitalization of New Horizon immediately following the closing of the Merger and the directors and management of HPI retained a majority of board seats and key positions in the management of New Horizon, HPI is considered to be the acquiring company for accounting purposes, and the transaction is being accounted for by New Horizon as a reverse acquisition under the acquisition method of accounting for business combinations. Accordingly, the acquisition consideration for accounting purposes consists of the New Horizon ordinary shares held by the historical shareholder of Vidara immediately following the completion of the Merger and the cash consideration payable to the shareholder of Vidara in redemption of bonus shares of Vidara. Assets and liabilities of Vidara will be measured at fair value and added to the assets and liabilities of HPI, and the historical results of operations of HPI will be reflected in the results of operations of New Horizon following the Merger.

The HPI balance sheet and statement of operations information as of and for the six months ended June 30, 2014 was derived from its unaudited consolidated financial statements included in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2014. The HPI balance sheet and statement of operations information as of and for the year ended December 31, 2013 was derived from its audited consolidated financial statements included in its Annual Report on Form 10-K for the year ended December 31, 2013.

The Vidara balance sheet and statement of operations information as of and for the six months ended June 30, 2014 was derived from its unaudited combined financial statements included in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2014, and the statement of operations for the year ended December 31, 2013 was derived from its audited combined financial statements included in the HPI proxy statement/prospectus filed on August 8, 2014. The AZ VIMOVO statement of operations information is derived from HPI’s unaudited pro forma condensed combined statement of income/(loss) for the year ended December 31, 2013 included in Exhibit No. 99.1 to HPI’s Annual Report on Form 10-K for the year ended December 31, 2013.

New Horizon has not completed a full, detailed valuation analysis necessary to determine the fair values of Vidara’s identifiable assets acquired and liabilities assumed. However, a preliminary valuation analysis was performed as of June 30, 2014, the date on which the proposed Merger is assumed for purposes of the pro forma balance sheet, related to developed technology, customer relationships, inventories, and contingent royalty liabilities.

A final determination of the fair value of Vidara’s assets and liabilities will be based on the actual net tangible and intangible assets and liabilities of Vidara that existed as of the date of completion of the Merger. Accordingly, the unaudited pro forma purchase price adjustments are preliminary and are subject to further adjustments as additional information becomes available and as additional analyses are performed, and such further adjustments may be material. The preliminary unaudited pro forma purchase price adjustments have been made solely for the purpose of providing the unaudited pro forma combined financial information presented below. HPI has estimated the fair value of Vidara’s assets and liabilities based on discussions with Vidara’s management, preliminary valuation studies and due diligence.

The number of New Horizon ordinary shares retained by Vidara Holdings was fixed in schedule 1 to the Merger Agreement as 31,350,000 ordinary shares of New Horizon and the fair value of these New Horizon ordinary shares used in these unaudited pro forma combined financial statements equals the closing per-share market value of HPI common stock of $12.29 as of September 17, 2014. The estimated amount of cash paid as consideration to Vidara Holdings at closing is $200,459,301 in these unaudited pro forma

combined financial statements. The actual cash will be determined as of the time of closing and the fair value of the New Horizon ordinary shares will be determined as of the time of closing. The terms of the debt financing entered into in order to pay Vidara Holdings cash in connection with closing will also be determined as of the time of closing. Accordingly, the unaudited pro forma combined financial statements include only preliminary estimates. The amounts of acquisition consideration, assets acquired and liabilities assumed that will be used in acquisition accounting will be based on their respective fair values as determined as of the time of closing, and may differ significantly from these preliminary estimates.

The unaudited pro forma combined financial statements do not give effect to the potential impact of current financial conditions, regulatory matters, operating efficiencies or other savings or expenses that may be associated with the acquisition. The unaudited pro forma combined financial statements also do not include any estimated integration costs. The unaudited pro forma combined financial statements have been prepared for illustrative purposes only and are not necessarily indicative of the financial position or results of operations in future periods or the results that actually would have been realized had HPI and Vidara been a combined company during the specified periods. The unaudited pro forma combined financial statements, including the notes thereto, should be read in conjunction with (1) the historical unaudited consolidated financial statements of HPI included in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2014, (2) the historical consolidated financial statements of HPI included in its Annual Report on Form 10-K for the year ended December 31, 2013, (3) the unaudited combined financial statements of Vidara included in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2014, and (4) the historical combined financial statements of Vidara for the year ended December 31, 2013 included in the HPI proxy statement/prospectus filed on August 8, 2014.

Unaudited Pro Forma Combined Balance Sheet

As of June 30, 2014

(In thousands)

See accompanying notes to the unaudited pro forma combined financial statements.

Unaudited Pro Forma Combined Statement of Operations

For the six months ended June 30, 2014

(In thousands, except per share amounts)

See accompanying notes to the unaudited pro forma combined financial statements.

Unaudited Pro Forma Combined Statement of Operations

For the year ended December 31, 2013

(In thousands, except per share amounts)

See accompanying notes to the unaudited pro forma combined financial statements.

Horizon Pharma plc

NOTES TO UNAUDITED PRO FORMA COMBINED FINANCIAL STATEMENTS

1. Description of Transaction

On March 18, 2014, Horizon Pharma, Inc. (“HPI”), Vidara Therapeutics Holdings LLC, a Delaware limited liability company (“Vidara Holdings”), Vidara Therapeutics International Ltd, a private limited company formed under the laws of Ireland (“Vidara”), Hamilton Holdings (USA) Inc., a Delaware corporation (“US HoldCo”), Hamilton Sub Inc., a Delaware corporation and wholly owned subsidiary of HoldCo (“Merger Sub”), entered into a Transaction Agreement and Plan of Merger (the “Merger Agreement”). Under the terms of the Merger Agreement, on September 19, 2014, HPI and Vidara combined their businesses in a stock transaction in which (a) Vidara carried out the reorganization described below and (b) Merger Sub merged with and into HPI, with HPI surviving as an indirect wholly-owned subsidiary of Vidara (the “Merger”). The transaction was approved by the boards of directors of both HPI and Vidara and by Vidara Holdings, the sole shareholder of Vidara prior to the reorganization.

Prior to the effective time of the Merger, Vidara carried out a reorganization of its capital structure (the “reorganization”). The reorganization consisted of a series of corporate actions as a result of which: (i) Vidara formed a new non-resident Irish company that is a tax resident in Bermuda referred to as Newco, (ii) Vidara assigned all of its contracts and has sold and transferred all of its intellectual property to Newco in exchange for a promissory note with an original principal amount equal to the fair market value of such assets, which was repaid in consideration for the issuance of two promissory notes of the same aggregate original principal amount, one of which was repaid in cash on the closing date, (iii) Vidara moved its tax residence from Bermuda to Ireland, (iv) Vidara created a new class of ordinary shares denominated in US dollars (as well as created additional euro-denominated share capital up to a par value of €40,000) such that the aggregate number of US dollar-denominated ordinary shares was sufficient to cover the ordinary shares issued in exchange for the outstanding shares of HPI common stock and shares of HPI common stock reserved for issuance under outstanding HPI equity awards and warrants and HPI’s outstanding convertible notes, and ordinary shares and bonus shares equal to 31,350,000 shares representing Vidara Holdings’ agreed shareholdings in New Horizon following the closing and a bonus issue of shares issued to Vidara Holdings and redeemed by Vidara from distributable reserves for cash as of the closing, (v) Vidara was re-registered as a public limited company in Ireland, (vi) Vidara redeemed the bonus issue of shares for cash in the amount of $200,000,000 plus cash on hand at Vidara on the closing date, less Vidara’s unpaid indebtedness and unpaid transaction expenses, with a further adjustment based on Vidara’s working capital as of the closing relative to a target working capital of $123,000 and (vii) Vidara was renamed Horizon Pharma plc.

At the effective time of the Merger, among other things, (i) each share of HPI common stock then issued and outstanding was canceled and automatically converted into and became the right to receive one ordinary share of New Horizon and (ii) each outstanding option, warrant or another equity award of HPI was converted into an option, warrant or another equity award of New Horizon with substantially the same terms and conditions, including the number of shares and the exercise price, as were applicable before the effective time of the Merger.

Pursuant to the Merger Agreement, as of the effective time of the Merger, the directors of New Horizon became the directors of HPI as of immediately prior to the effective time of the Merger, plus one additional director designated by Vidara, and the officers of New Horizon following the Merger were designated by HPI.

On November 18, 2013, HPI entered into an asset purchase agreement with AstraZeneca AB (“AstraZeneca”), pursuant to which HPI agreed to acquire from AstraZeneca and its affiliates certain intellectual property and other assets, and assume from AstraZeneca and its affiliates certain contingent liabilities, each with respect to VIMOVO^®. In connection with the closing of the VIMOVO Acquisition, on November 22, 2013, a letter agreement with AstraZeneca and Pozen Inc. (“Pozen”) became effective. AstraZeneca assigned to HPI its amended and restated collaboration and license agreement for the United States with Pozen, as amended, and HPI entered into a license agreement with AstraZeneca, a transition agreement with AstraZeneca and a supply agreement with AstraZeneca’s affiliate, AstraZeneca LP. HPI has accounted for this transaction as a business combination in accordance with Financial Accounting Standards Board Accounting Standards Codification (“ASC”) 805, “Business Combinations”.

In addition, on November 22, 2013, HPI closed on an offering of $150 million aggregate principal amount of 5.00% Convertible Senior Notes due 2018 (the “5.00% Convertible Notes”) pursuant to note purchase agreements (the “Note Purchase Agreements”) entered into by and between HPI and the purchasers of the Notes (the “Purchasers”). In connection with the closing, on November 22, 2013, HPI issued and sold to the Purchasers the 5.00% Convertible Notes pursuant to the Note Purchase Agreements. The 5.00% Convertible Notes are governed by an Indenture, dated as of November 22, 2013 (the “Indenture”), between HPI and U.S. Bank National Association, as trustee (the “Trustee”). HPI used $70.4 million of the net proceeds from the sale and issuance of the 5.00% Convertible Notes to repay all obligations under HPI’s previous senior secured loan as of November 22, 2013.

On November 22, 2013, HPI also entered into capped call transactions to cover, subject to anti-dilution adjustments substantially similar to those applicable to the 5% Convertible Notes, the number of shares of HPI’s common stock underlying the

5.00% Convertible Notes. The capped call transactions were entered into to reduce potential dilution to HPI’s common stock upon any conversion of the 5.00% Convertible Notes in excess of the principal amount of converted 5.00% Convertible Notes if the market price per share of HPI’s common stock, as measured under the terms of the capped call transactions, is greater than the strike price of the capped call transactions, which initially corresponds to the conversion price of the Notes and is subject to anti-dilution adjustments substantially similar to those applicable to the conversion rate of the Notes. HPI expects the options that are part of the capped call transactions to be exercised shortly prior to the maturity date of the Notes and will settle on or about the maturity date of the 5.00% Convertible Notes. HPI will not be required to make any cash payments to the option counterparties or their respective affiliates upon the exercise of such options, but will be entitled to receive from the option counterparties a number of shares of HPI’s common stock generally based on the amount by which the market price of HPI’s common stock, as measured under the terms of the capped call transactions, is greater than the strike price of the capped call transactions during the relevant settlement averaging period under the capped call transactions. Results for the VIMOVO Acquisition are included in HPI’s historical results of operations from the acquisition date.

2. Basis of Presentation

Because HPI security holders owned approximately 74% of the ordinary shares of New Horizon on a fully diluted basis immediately following the closing of the Merger, and the directors and management of HPI retained a majority of board seats and key positions in the management of New Horizon, HPI is deemed to be the acquiring company for accounting purposes, and the transaction is being accounted for by HPI as a reverse acquisition under the acquisition method of accounting for business combinations. Accordingly, assets and liabilities of Vidara will be measured at fair value and added to the assets and liabilities of HPI, and the historical results of operations of HPI will be reflected in the results of operations of New Horizon following the Merger.

The unaudited pro forma combined financial information was prepared using the acquisition method of accounting, based on the historical financial statements of HPI and the Vidara Group, with HPI being the accounting acquirer. Certain reclassifications have been made to the historical financial statements of the Vidara Group to conform to the financial statement presentation adopted by the combined company. These adjustments are related to the presentation of customer deposits, accrued trade discounts and rebates, accrued royalties, depreciation and amortization, royalty expense, selling expenses, and miscellaneous expenses. All such adjustments and reclassifications have been included in the pro forma adjustments in the Unaudited Pro Forma Combined Balance Sheet and Unaudited Pro Forma Combined Statements of Operations.

The total estimated acquisition consideration for the acquisition (for accounting purposes) of Vidara is expected to equal the market value of the New Horizon ordinary shares that were held by current Vidara shareholders immediately following the closing of the Merger plus the estimated cash consideration. For purposes of these unaudited pro forma combined financial statements, the acquisition consideration was based on the number of New Horizon ordinary shares that would have been held by the current Vidara shareholder, had the Merger closed on a date prior to the actual closing date, specifically, utilizing the September 17, 2014 stock price of $12.29, and the market value of HPI common stock as of that date ($385,291,500) and cash of $200,459,301 based on estimates had the Merger closed on that prior date. Cash consideration was estimated to be $200,000,000, prior to a working capital adjustment. Total acquisition consideration as of this date was estimated to be $585,750,801. An increase of 10% in HPI’s share price would have increased the total consideration by $38,529,150, and a decrease of 10% in HPI’s share price would have decreased the total consideration by $38,529,150.

Under the acquisition method of accounting, identifiable assets and liabilities of Vidara, including identifiable intangible assets, will be recorded based on their estimated fair values as of the date of the closing of the Merger. The excess of the fair value of the net assets acquired over the value of consideration will be recorded as an initial bargain purchase gain.

The pro forma adjustments to reflect the acquired assets and assumed liabilities of Vidara are based on the fair value of Vidara’s assets and liabilities as of June 30, 2014. The estimated acquisition consideration and the preliminary allocation of the estimated acquisition consideration are, in part, based upon a preliminary management valuation, as described below, and HPI and Vidara’s estimates and assumptions which are subject to change.

Cash and cash equivalents, and other tangible assets and liabilities: Tangible assets and liabilities were valued at their respective carrying amounts, except for adjustments to inventories and prepaid assets. HPI and Vidara believe that these amounts approximate their current fair values.

Inventories: Inventories acquired included raw materials and finished goods. Fair value of finished goods has been determined based on the estimated selling price, net of selling costs and a margin on the selling costs. Fair value of raw materials has been estimated to equal the replacement cost.

Identifiable intangible assets and liabilities: Identifiable intangible assets and liabilities acquired included developed technology and customer relationships. The fair value of intangible assets is based on management’s preliminary valuation. Estimated useful lives

(where relevant for the purposes of these unaudited pro forma combined financial statements) are based on the time periods during which the intangibles are expected to result in incremental cash flows.

Developed technology: Developed technology intangible assets reflect the estimated value of Vidara’s rights to the currently marketed product. The fair value of developed technology was determined using an income approach. The income approach explicitly recognizes that the fair value of an asset is premised upon the expected receipt of future economic benefits such as earnings and cash inflows based on current sales projections and estimated direct costs for Vidara’s product line. Indications of value are developed by discounting these benefits to their present worth at a discount rate that reflects the current return requirements of the market. The fair value of developed technology will be capitalized as of the acquisition date and subsequently amortized over the estimated remaining life of the product at 15 years.

Customer relationships: Customer relationships intangible assets reflect the estimated value of Vidara’s customer base. Vidara’s customers as of the acquisition date were predominantly retail pharmacies with demand for Actimmune, which is a small population. As such, a significant portion of revenue growth is expected to be generated from customers existing as of the acquisition date. Management assessed the historical customer trends to identify the anticipated attrition. The fair value of customer relationships will be capitalized as of the acquisition date and subsequently amortized over the estimated remaining life of the product at 10 years.

Bargain purchase gain: Bargain purchase gain represents the excess of the estimated fair values of net assets acquired over the preliminary acquisition consideration. The bargain purchase gain has been reflected in the unaudited pro forma combined balance sheet as an adjustment to retained earnings. However, the bargain purchase gain has not been reflected in the unaudited pro forma combined statements of operations as it is a non-recurring item that is directly related to the Merger.

Deferred tax assets and liabilities: Deferred tax assets and liabilities arise from acquisition accounting adjustments where book values of certain assets and liabilities differ from their tax bases. Deferred tax assets and liabilities are recorded at the currently enacted rates which will be in effect at the time when the temporary differences are expected to reverse in the country where the underlying assets and liabilities are located (United States or Ireland). Vidara’s customer relationships intangible assets as of the acquisition date were located in the United States where a U.S. tax rate of 39% is being utilized and a deferred tax liability is recorded. Vidara’s developed technology assets as of the acquisition date were located in Bermuda which does not have income taxes; accordingly, no deferred tax liabilities were recorded related to developed technology assets.

Pre-existing contingencies: HPI has identified a contingent liability potentially payable under previously existing royalty and licensing agreements. The initial fair value of this liability was determined using a discounted cash flow analysis incorporating the estimated future cash flows of royalty payments based on future sales. The liability will be periodically assessed based on events and circumstances related to the underlying milestones, and any change will be recorded in New Horizon’s consolidated statement of operations.

The preliminary determination of the fair value of the acquired net assets, assuming the Merger had closed on June 30, 2014, is as follows (in thousands):

HPI accounts for business combinations in accordance with Financial Accounting Standards Board Accounting Standards Codification (“ASC”) 805, “Business Combinations”. The purchase price for the acquisition has been assigned to the assets and

liabilities acquired based on a preliminary valuation of their respective fair values. Upon completion of detailed valuation studies New Horizon may make additional adjustments to the fair values, and these valuations could change significantly from those used to determine certain adjustments in the pro forma combined statement of earnings.

On June 17, 2014, HPI, as initial signatory, entered into a Credit Agreement (the “Credit Agreement”) with the lenders from time to time party thereto (each a “Lender” and collectively the “Lenders”) and Citibank, N.A., as administrative agent and collateral agent.

The Credit Agreement provides for (i) a committed five-year $300 million term loan facility (the “Term Loan Facility”), the proceeds of which are required to be used to effect the transactions contemplated by the Merger Agreement, to pay fees and expenses in connection therewith and for general corporate purposes, (ii) an uncommitted accordion facility subject to the satisfaction of certain financial and other conditions, and (iii) one or more uncommitted refinancing loan facilities with respect to loans under the Credit Agreement.

The borrower under the Term Loan Facility is U.S. HoldCo. The Credit Agreement allows for Vidara and other subsidiaries of Vidara to become borrowers under the accordion facility. Loans under the Term Loan Facility bear interest, at each Borrower’s (as such term is defined in the Credit Agreement) option, at a rate equal to either the LIBOR rate, plus an applicable margin of 8.00% per annum (subject to a 1.00% LIBOR floor), or the prime lending rate, plus an applicable margin equal to 7.00% per annum.

3. Accounting Policies

Following the acquisition, New Horizon will conduct a review of Vidara’s accounting policies in an effort to determine if differences in accounting policies require adjustment or reclassification of Vidara’s results of operations or reclassification of assets or liabilities to conform to HPI’s accounting policies and classifications. As a result of that review, New Horizon may identify differences between the accounting policies of the two companies that, when conformed, could have a material impact on these pro forma combined financial statements. During the preparation of these pro forma combined financial statements, HPI was not aware of any material differences between accounting policies of the two companies, except for certain reclassifications necessary to conform to HPI’s financial presentation, and accordingly, these pro forma combined financial statements do not assume any material differences in accounting policies between the two companies.

Pro Forma Adjustments

4. Vidara—Balance Sheet Reclassification

Certain reclassifications have been recorded to Vidara’s historical balance sheet to conform to HPI’s presentation, as follows (in thousands):

5. Vidara—Statement of Operations Reclassification

Certain reclassifications have been recorded to Vidara’s historical statements of operations to conform to HPI’s presentation, as follows (in thousands):

Year ended December 31, 2013

Six months ended June 30, 2014

6. Pro Forma Adjustments—Balance Sheet

The preliminary bargain purchase gain results from the estimated fair value of Vidara’s identifiable net assets acquired exceeding the estimated purchase price. The preliminary bargain purchase gain will be adjusted based upon the final accounting as of the effective date of the Merger. The preliminary bargain purchase gain has been reflected in the unaudited pro forma combined balance sheet as an adjustment to retained earnings. However, the bargain purchase gain has not been reflected in the unaudited pro forma combined statements of operations as it is a non-recurring item that is directly related to the Merger. In accordance with ASC 805, the bargain purchase gain of $9.2 million will be recognized as of the acquisition date.

ASC 820, Fair Measurements and Disclosures, defines fair value, establishes the framework for measuring fair value for any asset acquired or liability assumed under U.S. GAAP, expands disclosures about fair-value measurements and specifies a hierarchy of valuation techniques based on the nature of the inputs used to develop the fair value measures. Fair value is defined in ASC 820 as “the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date”. The fair value of identifiable intangible assets (Developed Technology) is determined primarily using the “income approach”, which is a valuation technique that provides an estimate of the fair value of an asset based on market participant expectations of the cash flows an asset would generate over its remaining useful life. Some of the more significant assumptions inherent in the development of the identifiable intangible assets valuations, from the perspective of a market participant, include the estimated net cash flows for each year for each project or product (including net revenues, cost of sales, research and development costs, selling and marketing costs and working capital/asset contributory asset charges), the appropriate discount rate to select in order to measure the risk inherent in each future cash flow stream, the assessment of each asset’s life cycle, competitive trends impacting the asset and each cash flow stream as well as other factors.

No assurances can be given that the underlying assumptions used to prepare the discounted cash flow analysis will not change or the timely completion of each project to commercial success will occur. For these and other reasons, actual results may vary significantly from estimated results.

7. Vidara—Pro Forma Adjustments—Statement of Operations