We don't have a WII yet, but I haven't spoken to anyone who doesn't love it. It's supposed to be extremely family friendly because it allows everyone to play together unlike other electronics where kids have their noses down and don't ever look up. It's probably like anything with the ratings, though. You just have to watch for the types of games they play just as we have to watch which t.v. shows are most appropriate. Personally, I still believe that nothing takes the place of the great outdoors. I would plan on creating time limits from day one if you get one. They get HOOKED! My son is now 9 and we got him a DS when he was a little younger. He LOVES it, but he would stay on it all day if I let him. I have to limit his time on it. What I like about it, though, is that he can take it on the go and play it in the car or at places like the waiting room in the doctor's office, etc.. I know, however, that he would trade his DS in anyday for a WII. I'm sure the WII is much more entertaining.
I have no doubt that a 4 year old can figure out how to play. They are so smart! I know a couple of people who have little ones that are able to play. Kids are so amazing the way they figure electronics out at such a young age! But, it might be a thought to give him something to look forward to when he's a little older. The saying is that if you give someone steak, they will never want or appreciate hamburger meat again. Lol! Progression is always a wonderful thing. If you do decide to get one, my advice is to go into a store and have him test out the controls to see if the size is compatible and if the games are appropriate for him.
Good luck! He sounds like a lucky little guy!