For a 4yo I would most likely buy the Leapster 2. My son, who is 6 now, had a V-smile (not handheld, we lost it in Ike though) that we got for him when he was 3 1/2. He had trouble at first understanding what he needed to do. However, in the last couple of years he had gotten to where he really enjoyed it. He had games up to 4-6 years. We had not gotten him any of the "older" games yet.
I liked the V-Smile because the games were educational. I have never been a fan of video games because I believe that they are one of the contributors to childhood obesity. My husband and I have decided that if we ever have a "big" video game system in our house it will be the Wii because you actually move around to play it.
Anyway, as for the DS. My son had never really been exposed to one until this summer. Before that he had played his cousin's, who is a year younger than he is. This summer I worked at and he attended a summer camp where the kids were allowed to bring theirs to play during free time. He played with them occasionally, but would watch the owners play with them too. He had asked for one, we told him maybe for Christmas.
Well, when we evacuated to my parents for Ike, everytime he asked his cousin to play it, she would decide she wanted it, and a fight would ensue. My dad felt bad for him, so he typed up a letter from "Santa" saying that he needed to do a test run for Christmas since there were so many boys and girls not living in their usual place because of Ike. The next day he had a DS waiting when he got home from school.
He mainly only plays it when we are on road trips. He's gotten in to playing video games online here lately. Disney Channel, Cartoon Network, etc. have video games on their web-sites. He does have some trouble with the DS occasionally because you have to read some stuff and he is not to that point yet. As for the smaller buttons, that doesn't bother him at all.
Long story short, for a 4yo I would get the Leapster or VSmile. Yes, the DS has games for little ones, but the Leapster and VSmile are geared for little ones. And by the time yours gets older there will probably be a newer version of the DS he would want anyway. Plus, the Leapster and VSmile are built with shock resistance if a little one drops it. The DS is more fragile.