President of the Senate, Senator Godswill Akpabio, read their separate letters in the chamber where they cited irreconcilable differences in the opposition PDP as reason for their cross-carpeting.
Akpabio, who read the letters, authorised that they should be documented by the Clerk of the Senate.
The Senators who decamped formally on Wednesday are Senators Francis Fadahunsi, Osun East and Oluwole Olubiyi, Osun Central.
Others are Senators Aniekan Bassey, Akwa Ibom North East and Samson Ekong, Akwa Ibom South.
Following the defection of the four Senators, the updated composition of the Senate now stands as follows: APC 70, PDP 28; Labour Party 5, NNPP 1, SDP 2 and APGA 1.
Following the defection, a few members of the House of Representatives, former Labour and Productivity Minister Senator Chris Ngige, and APC National Secretary Senator Ajibola Basiru were present in the sacred chamber to show support for the party.