Let's get the obligatory bad news out of the way: there is no four-player co-op for the campaign mode in Gears of War 2.
Players will just have to make due with the traditional two-player mode, presumably starring Marcus Fenix and Dominic Santiago as the only two playable characters. Thankfully Epic is bringing a mode to Gears 2 that some are sure to enjoy even more than playing through the campaign with a friend. It's called Horde and it allows for five human players to link up and do battle against wave after wave of AI-controlled enemies. IGN played it, it's awesome and it's going to ruin what little social life they have left.
Not only did Epic take the veil off a brand new mode, but IGN also got to play on two new maps with new weapons and some fantastic enemies that change the Gears experience ever so slightly.