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http://www.esrb.org/ratings/ratings_guide.jsp
C'mon man, he is FOUR already and you only give him Teen rated games.
Next year you gotta get him up to Mature level with games like GTA IV and Fallout 3 so that his buddies won't make fun of him.... 
We don't let him play bf2, only once in a while when I'm playing too and let him have a turn. But I don't think Star Wars Battlefront 2 or Transformers are that bad. He also plays AH offline with my help.
He has asked to play Fallout 3 already but got the definite no on that

If you can find me suitable Early Childhood rated games then I'll let him loose. But to date I can't find anything other than flash based rubbish that looks like it was written in the 1980's.
The problem is young kids need 'sandbox' style games and they are few and far between. I think many people underestimate their ability to learn stuff, my guy regularly works out things I didn't know you could do in games and he can't even RTFM.