"Metagame". Under this barbaric name hides a feature allowing you to spice-up the campaign mode. By default, it rewards the player according to its performance during combat. Speed, Accuracy and other criterion (combos, number of ennemies killed) influence your final score that is put on line and compared to other players'.
More importantly, the metagame also applies to co-op, all the players then compete against one another (without hindering each other, treason being heavily punished) to obtain the best score at the end of the mission. Lastly, skulls (hidden in the levels and activated before the mission) are handicaps that strongly reward you with points. One of them will warp everyone to the last checkpoint upon the death of one of the players. Harsh!
There is no mention of 4 player co-op in this blurb, and the only place where it is mentioned doesn't say that they have seen it, just that it appears to be featured on certain screenshots.