Viva Pinata: Party Animals is a sort of Mario Party-like game that is 4 player. Otherwise, I'd pretty much stick with Halo 3. It offers a lot to do with 4 players on the same system, and you can even bring 4 players online to play. My personal favorite is infection; we turn zombie speed up to 300% making them almost as fast as warthogs and increasing their damage and health regeneration speed.
Lets say one 360 is in AV2 and one in AV3. Yes, you can system link, but you would only be able to display one 360 at a time on the TV. To display the other, you would need to switch from AV2 to AV3 (or vice versa). So yes, you could but it would be no fun unless its a game like Civ Revolution where you don't do things simultaneously.