You can only play 2 player co-op on one Xbox 360 console. To be able to play with 4 players in co-op, you will need to have 4 Xbox 360 consoles connected through system link. Once system link has been enabled, you can no longer do split screen play. Therefore, you cannot have 4 players with 2 Xbox 360 consoles that are system linked (split screen per Xbox 360). You will need to have 4 Xbox 360 consoles.
The breakdown is as follows:
1 Xbox 360 console = 2 players split screen
4 Xbox 360 consoles = 4 players co-op/terrorists hunt (system link)