UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 4)*

UNION FIRST MARKET BANKSHARES CORPORATION

(Name of Issuer)

Common Stock, no par value

(Title of Class of Securities)

905399101

(CUSIP Number)

D. Michael Jones

Markel Corporation

4521 Highwoods Parkway

Glen Allen, Virginia 23260

(804) 747-0136

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

August 5, 2013

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because § 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g) check the following box  ¨.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act.

SCHEDULE 13D

Note: This Amendment No. 4 amends and supplements the Schedule 13D originally filed with the Securities and Exchange Commission on February 11, 2010, as amended on February 13, 2013, March 7, 2013 and July 1, 2013. Capitalized terms used in this Amendment No. 4 and not defined herein have the respective meanings ascribed to such terms in the Schedule 13D, as previously amended.

Item 5 is hereby amended and restated as follows:

(a) As of the date hereof, Markel beneficially owns 1,189,214 shares of the Issuer’s common stock, which represents approximately 4.8% of the outstanding shares of the class. The percentage calculation is based on 24,885,653 shares outstanding, which reflects the number of shares outstanding as of August 1, 2013, as reported by the Issuer. The ownership of the Issuer’s common stock by the Listed Persons is provided on Schedule A annexed hereto and is incorporated herein.

(b) Markel has sole voting and dispositive power with regard to the shares of common stock described in Item 5(a).

(c) Schedule B annexed hereto lists all the transactions in the Issuer’s common stock by Markel and Steven A. Markel during the past 60 days. There were no transactions in the Issuer’s common stock by Thomas S. Gayner during the past 60 days.

(d) No person other than Markel is known to have the right to receive, or the power to direct the receipt of, dividends from, or proceeds from the sale of, the shares of common stock reported in this Statement.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: August 12, 2013

SCHEDULE A

Each of the above named persons is a U.S. citizen.

The principal business address for each of the persons listed above is c/o Markel Corporation, 4521 Highwoods Parkway, Glen Allen, Virginia 23060.

SCHEDULE B

The sales by Markel Corporation dated June 17 through June 25 listed in the table above were previously reported in Amendment No. 3 to the Company’s Schedule 13D filed with the Securities and Exchange Commission.