The truth is that there aren't very many good four player games on the PS3. Even games like Modern Warfare have some serious problems.
Modern Warfare outputs at 600p, giving each player a tiny number of pixels when you divide by four. Then Modern Warfare maps tend to be very busy. There's lots of stuff sitting around and in MW2 there's lots of effects (snow, dust, etc). Personally, I only like to play the map "Rust" when I'm playing with three others.
Rock Band is a fun four player game (or six players with the Beatles Rock Band). You may want to look on PSN for some downloadable games, such as "Calling All Cars."
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