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.
On-console PS3 show FirstPlay has issued a correction regarding this week's Killzone 3 preview.
It reads: "Unfortunately the Killzone 3 preview contains a piece of erroneous information about a four-player co-op mode which isn't present in the game. It was something that had been rumoured that we mistakenly picked up on as fact. Apologies for that, especially to Guerrilla for whom this has been an annoyance."