Intuous 4 without a doubt!! I was borrowing my mate's 3 and thought it was time to buy a 4. Bought the 4 and was soooo much happier. The scrolling touchpad is like the scroll features of a middle mouse button. Especially in maya it's soo useful!
I have an Intuos 3 and bought my wife an Intuos 4. As far as I can tell, the major difference is the hotkey controls are better on the Intuos 4. My wife hardly ever even uses them.
~ I use an Intuos 3 at home, and an Intuos 4 at work. The difference between them is minimal enough for me to advise you to get the Intuos 3, especially because the nibs seem to last forever in comparison to the 4.
I'd get whatever you can find cheapest, really don't see the point in any of the extra stuff on the Intuos 4. Then again I still have an Intuos 1 at home and I find that's just fine as well (but I've used all 4 generations at work). :)