Omnicom plans to lay off over 4,000 employees and consolidate several well-known advertising agency brands following its $13.5 billion acquisition of rival Interpublic Group (IPG), according to interviews with company executives.

The report indicates that DDB, a creative agency founded in 1949, and MullenLowe, a creative marketing agency, will be merged into TBWA, a subsidiary of Omnicom.

Omnicom has not yet responded to requests for comment.

Additionally, FCB—one of IPG’s largest global advertising networks, with roots dating back to 1873—will be integrated into Omnicom’s BBDO.

Omnicom CEO John Wren stated that as part of the IPG integration plan, the company will eliminate more than 4,000 positions, primarily in administrative roles but also including some management positions.

The report has not been independently verified.