UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K/A

Amendment No. 4 to

CURRENT REPORT

Pursuant to Section 13 or 15(d) of

The Securities Exchange Act of 1934

Date of Report (Date of Earliest Event Reported): April 29, 2011

POWER SOLUTIONS INTERNATIONAL, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (630) 350-9400

Copies to:

Katten Muchin Rosenman LLP

525 W. Monroe Street

Chicago, IL 60661

Tel.: (312) 902-5200

Fax: (312) 902-1061

Attn: Mark D. Wood, Esq.

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Explanatory Note

On May 5, 2011, Power Solutions International, Inc. (f/k/a Format, Inc.) (the “Company”) filed with the Securities and Exchange Commission (the “SEC”) its original Current Report on Form 8-K (the “Original Form 8-K”; the Original Form 8-K as amended by Amendment No. 1 to the Original Form 8-K, filed with the SEC on May 16, 2011, as further amended by Amendment No. 2 to the Original Form 8-K, filed with the SEC on May 20, 2011, and as further amended by Amendment No. 3 to the Original Form 8-K (“Amendment No. 3), filed with the SEC on August 1, 2011, the “Amended Form 8-K”) to report certain events described in detail therein, including, among other things, (1) the completion of a reverse recapitalization transaction on April 29, 2011 involving the Company and The W Group, Inc. (“The W Group”), as a result of which the Company has succeeded to the business of The W Group, and (2) the Company’s consummation of a private placement of shares of its Series A Convertible Preferred Stock and warrants to purchase shares of the Company’s common stock, from which the Company received $18.0 million in gross offering proceeds, before payment of commissions, fees and expenses.

The Company previously filed partially redacted copies of the Supply Agreement, dated December 11, 2007, by and between PSI International, LLC and Doosan Infracore Co., Ltd., as amended, as Exhibit 10.12 to both the Original Form 8-K and Amendment No. 3. This Amendment No. 4 to the Amended Form 8-K (“Amendment No. 4”) is being filed by the Company in connection with its withdrawal of its application for confidential treatment with respect to some, but not all, redacted portions of this exhibit for which the Company had previously sought confidential treatment pursuant to applications under Rule 24b-2 under the Securities Exchange Act of 1934 and Rule 406 under the Securities Act of 1933. A revised redacted copy of this exhibit that includes previously redacted information as to which the Company has withdrawn its request for confidential treatment is attached as Exhibit 10.12 to this Amendment No. 4.

Except as described above and set forth herein, no modifications have been made to information contained in the Amended Form 8-K, and the Company has not updated any information contained therein to reflect events that have occurred since the date of the Original Form 8-K (except as expressly set forth in Amendment No. 1, Amendment No. 2 or Amendment No. 3 thereto). This Amendment No. 4 should be read in conjunction with the Amended Form 8-K.

(a) Financial Statements: The W Group’s audited financial statements for the fiscal years ended December 31, 2008, 2009 and 2010 and The W Group’s unaudited consolidated financial statements as of March 31, 2011 and for the three months ended March 31, 2011 and 2010 have been previously included in this Current Report on Form 8-K.

(b) Pro forma financial information: The required unaudited pro forma consolidated financial statements with respect to the Company and The W Group have been previously included in this Current Report on Form 8-K.

(d) Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.