I kinda feel that giving a 4 year old games is a bad idea. I mean, when I was four I had a huge imagination and used to just love playing with friends and making things up and doing what four year olds do. Then I remeber when I was like 6/7 getting a nintendo and spending hours and hours every day on it and having people round to play on it and pretty much everything in my life from then on was around games till I hit 17 haha. What im trying to say is that I think you should introduce him to games later. Not to menchen that children can get ridiculously addicted to them and become such a pain if they don't get to play them. I wasn't like that, though I verged on the edge sometimes.
Having said that, I do encourage learning ones. I had them as a kid. They weren't too bad.