My group has played most of the same games that you did. Here are the others we played:
- Killing Floor. It doesn't have the cinematics that L4D does, but it incorporates a few RPG concepts such as gaining rank and earning money for better weapons. Well received.
- Minecraft. Construction game that appeals strongly to explorer/builder types.
- League of Legends. Isometric arena game with a bazillion different classes. There was a free open beta but the game could be released now.
- Borderlands DLC. Worth a special mention because we ended up playing through every DLC. It's a bit expensive ($5 per right on steam sale) but the DLC really does add a lot of hours to the game and some of the fights are pretty well done (Cramerax).
- Dawn of War 2. Campaign is only co-op for 2, but it's a great game to play multiplayer with 4.
- HAWX 2. Pretty good chance of pace flight combat game, with an emphasis on arcade combat rather than simulation. 4 person co-op.
Upcoming co-op games:
Brink (up to 8)
Diablo 3
Bulletstorm