and as a father of a 4 yr old, I would definitely recommend the Xbox Kinect . The Wii is fun too and makes sense if you are a fan of "Ninetendo" only titles (Mario,Final Fantasy,Zelda etc).
But if the main purpose is to ensure that your son is physically involved in the game, then the Kinect it is. With the Wii its easy to "cheat" and not move much since you have a controller .Not so with the Kinect, since your body is the controller.

That said, a gaming console is no substitute for time spent actually doing something outdoors.If you are in an apartment building with no play-area and/or not enough kids to play with - have you considered other activities like enrolling him in a swimming class, tennis lessons maybe,go cycling on the corniche/parks a couple times a week ?
Maybe you already do some or all of what I'm suggesting above, in which case that gaming console isn't a bad thing