It holds data required for "Zombie" or "MP" (Multiplayer) modes.
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
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. It holds data required for "Zombie" or "MP"