Yesterday Microsoft surprised us when they announced Crackdown 2 for the Xbox 360 at their E3 2009 conference. At the time there were very few details of the game available, and we were unsure if the co-op mode would expand beyond the first's two player drop-in/drop-out.
The Official Crackdown 2 page has popped up on Xbox.com and it appears that the game will indeed have four player co-op. On top of that the page says Crackdown 2 will have 16 player multiplayer modes, which we assume would be team based death match and objective style games.