UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 4)*

COMSCORE, INC.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

20564W105

(CUSIP Number)

WPP plc

27 Farm Street

London, United Kingdom W1J 5RJ

Telephone: +44(0) 20 7408 2204

Attention: Andrea Harris, Esq.

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

Copies to:

Curt Myers, Esq.

Davis & Gilbert LLP

1740 Broadway

New York, New York 10019

(212) 468-4800

January 29, 2016

(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 of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ¨

NOTE: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §§ 240.13d-7 for other parties to whom copies are to be sent.

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 (however, see the Notes).

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This amendment No. 4 relates to the Schedule 13D filed by Cavendish Square Holding B.V., a private limited liability company incorporated under the laws of the Netherlands (“Cavendish”), and WPP plc, a corporation formed under the laws of Jersey with the Securities and Exchange Commission on April 7, 2015, as amended (the “Schedule 13D”), relating to the common stock, par value $0.001 per share (the “Common Stock”), of comScore, Inc., a Delaware corporation (the “Company”). Unless set forth below, all Items are unchanged from the Schedule 13D. Capitalized terms used herein and not otherwise defined herein shall have the meanings ascribed to such terms in the Schedule 13D.

Item 2. Identity and Background

Item 2 of the Schedule 13D is hereby amended and supplemented by adding the following:

WPP, plc and its subsidiaries (the “WPP Group”) comprise one of the largest communications services businesses in the world. The WPP Group provides communications services on a national, multinational and global basis. It operates from over 3,000 offices in 111 countries including associates. The WPP Group organizes its businesses in the following areas: Advertising and Media Investment Management; Data Investment Management; Public Relations & Public Affairs; and Branding & Identity, Healthcare and Specialist Communications (including direct, digital, promotion and relationship marketing).

The address of the principal office of Cavendish is Laan op Zuid 167, 3072 DB Rotterdam, Netherlands. The address of the principal office of Lux Gamma Three is 124 boulevard de la Pétrusse, Luxembourg, L-2330. The address of the principal office of WPP plc is 27 Farm Street, London, United Kingdom W1J 5RJ.

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Item 3. Source and Amount of Funds and Other Consideration.

Item 3 of the Schedule 13D is hereby amended and supplemented by adding the following:

“Between January 11, 2016 and January 15, 2016, Cavendish acquired an aggregate of 165,279 additional shares of the Common Stock in open market transactions effected through a broker-dealer (the “Additional Shares”). The consideration used by Cavendish to acquire the Additional Shares was Cavendish’s working capital.”

“Pursuant to the Merger Agreement (as defined below) upon consummation of the Merger (as defined below) each outstanding share of common stock of Rentrak Corporation (“Rentrak”) was converted into the right to receive 1.1500 shares of Common Stock of the Company, as described in Item 4 (which Item 4 is incorporated herein by reference). Prior to the completion of the Merger, Lux Gamma Three held 3,037,889 shares of Rentrak common stock (such shares, the “WPP Rentrak Shares”), and on January 29, 2016, upon completion of the Merger, Lux Gamma Three acquired an aggregate of 3,493,572 shares of Common Stock of the Company (the “Lux Gamma Three Shares”) in exchange for the WPP Rentrak Shares.”

Item 4. Purpose of Transaction

Item 4 of the Schedule 13D is hereby amended and supplemented by adding the following:

“On September 29, 2015, the Company entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Rentrak and Rum Acquisition Corporation, an Oregon corporation and a wholly owned subsidiary of the Company (“Merger Sub”), pursuant to which, at the effective time of the Merger (as defined below), Merger Sub would merge with and into Rentrak, with Rentrak surviving the merger as a wholly owned subsidiary of the Company (the “Merger”). On January 29, 2016, upon completion of, and as a result of the Merger, each outstanding share of Rentrak common stock was converted into the right to receive 1.1500 shares of Common Stock of the Company, which resulted in Lux Gamma Three acquiring the Lux Gamma Three Shares referred to in Item 3 (which Item 3 is incorporated here by reference) on such date. The foregoing description of the Merger Agreement is a summary only and is qualified in its entirety by reference to the full text of the Merger Agreement, which was filed as Exhibit 2.1 to the Form 8-K filed by the Company with the Securities and Exchange Commission on September 29, 2015 and is hereby incorporated by reference in response to this Item 4.”

Item 5. Interest in Securities of the Issuer.

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Item 7. Material to be Filed as Exhibits

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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: January 29, 2016

Annex A

Directors and Executive Officers of the Reporting Persons

The name, citizenship, position, principal occupation or employment and business address of the directors and executive officers of each of the Reporting Persons are set forth below.

1. CAVENDISH SQUARE HOLDING B.V.

2. WPP LUXEMBOURG GAMMA THREE S.À R.L.

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3. WPP

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Annex B

1. CAVENDISH SQUARE HOLDING B.V.

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Exhibit 1

JOINT FILING AGREEMENT

AGREEMENT dated as of January 29, 2016, by and among Cavendish Square Holding B.V., WPP Luxembourg Gamma Three S.à r.l. and WPP plc (collectively, the “Parties”).

Each of the Parties hereto represents to the other Parties that it is eligible to use Schedule 13D to report its beneficial interest in the common stock of comScore, Inc. (“Schedule 13D”) and it will file the Schedule 13D on behalf of itself.

Each of the Parties agrees to be responsible for the timely filing of the Schedule 13D and any and all amendments thereto and for the completeness and accuracy of the information concerning itself contained in the Schedule 13D, and the other Parties to the extent it knows or has reason to believe that any information about the other Parties is inaccurate.