If you try to launch zombies or multiplayer without this file, the game will likely close or throw an error indicating missing content.
It holds data required for "Zombie" or "MP" (Multiplayer) modes. fg-optional-mp-zombie-04.bin
It must be placed in the same folder as the setup.exe and other bin files before starting the installation. If you can tell me: Which Call of Duty game this is (e.g., Black Ops 2, MW3)? Are you trying to play local zombies or online multiplayer ? If you try to launch zombies or multiplayer