It doesn't matter if he's turning 4 or 74, everybody has a grand ol' time at Chef Mickey's! We've been there 6 times since 2000 and it never fails to amaze us! It's the only character meal (and the only meet n greet in all of WDW, if I'm correct) that you'll find all of the Fab Five in the same place for a greeting. The official WDW guidebook describes Chef Mickey's as "a celebration" and "best all-around character meal". Also every 45 minutes there's a big celebration where everybody dances and waves their napkin in the air.
Keep in mind, though, wherever you choose, your child will get a little something special from the characters. We had a member of our party last time have a birthday, and at each of the 3 character meals we attended (Chef Mickey's, OHana, CRT), they got a special desert and signed certificate by the characters. Good luck choosing and have a happy birthday!