In this week's edition of First Play it was confirmed that Killzone 3 will indeed feature four player co-operative play as has previously been rumoured.
"we do know that Guerrilla is working on a special objective-based 4-man co-op mode to go with the standard 2-player stuff."
Guerrilla Games have been very secretive about the co-op component up until now, but they have promised it will be special. Special enough for four it would seem.