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 without this file, the game will likely close or throw an error indicating missing content. fg-optional-mp-zombie-04.bin
It holds data required for "Zombie" or "MP" (Multiplayer) modes. It must be placed in the same folder as the setup
fg-optional-mp-zombie-04.bin is an used in FitGirl repacks, typically for Call of Duty games, that contains specific assets for multiplayer or zombie modes. Black Ops 2
You do not need to download this file if you only intend to play the single-player campaign.