Space Marine has finally received its four player co-op survival mode, Exterminatus. An automatic Steam update should add the mode to the Online section of the Space Marine main menu, and will offer you and four friends two maps on which to trounce hordes and hordes of Orks. I jumped in for five minutes and killed about 115 with nothing but a Heavy Bolter and a big power-armoured booty. Stomp, stomp! Dakka dakka! Weapon unlocked!
Experience earned popping green monsters in survival mode will also count towards your overall multiplayer rank, giving you new weaponry with which to embarrass your enemies, which acts as an unnecessary extra motivation to team up and make a big gooey mess of a charging Ork Waaaagh!