I guess it depends on what you are going to use your sound system for... if you are going to use it for normal music playing, and some games, 4.1 would be ok...
But if you are going to be playing any DVD movies, get 5.1 for sure to get the full surround sound...
In my opinion, with 5.1 home threater kits at such low prices these days, why buy 4.1 now? get 5.1 so you don't have to upgrade later...