With regards to four-player couch games, I wanted to mention the technical hurdles of pulling that off. When I play Mario Kart Wii, it looks noticeably worse when you have two people on a screen, let alone four. Effectively, the single-screen game is 60 fps, but that gets divided in half when you go to two player. I remember the same thing happening with CoD: World at War, Killzone 3 and Resistance 2 when I'd try playing with two people on a screen.
So, I don't know how the engines work, but it seems like there are definitely sacrifices to be made in the game experience when you try to push your console to provide single-player graphics to four-player screens.