4 on 4 is just the co-op. That is a first for the series since halo 2 only had 2 player co-op. Halo 2 was frustrating because you weren't able to pick the server you wanted to play, and they just automatically threw you in somewhere based upon a stupid ranking system. Max on the servers were 16 only and the system threw you into the same maps over and over at times.
If they give you more freedom in this one like an online pc game (can't wait for unreal tournament 3, the real juggernaut) then this might be a go. But, if it's trying to be too smart with the server match, then no thanks.