The Wii isn't poerful enough to do anything but the older GTA engine (GTA3, Vice City, San Andreas). In fact I don't think it's even powerful enough to do those games, so forget about GTAIV ever making it to the Wii.
They could probably do Chinatown Wars but it's sold so poorly (for a AAA title) that I doubt it will ever happen.
It is powerful enough to render GTA3.
But I don't think we'll ever see a GTA on the wii. The maximum I could imagine myself is chinatown wars in the form of wiiware.
Play with Scarface or The Godfather