What, GT4? No it will be more like GTC than anything, pick a car and race it and then the school mode, GT4 is only a demo of GT4. If your after info on GT4 though, that will be a huge game with over twice as many tune up options as GT3 and plenty more cars and tracks and some extra features but you'll have to check the GT4 forums for info on all thoes as theres far too much for me to list here.
Yes - you see what Sony/PD have done is periodically released attractive screenshots of a variety of cars, but all they've done over the last 3 years is play pool and go to strip clubs. In October/November (whenever it comes out), they'll just be releasing GT3 again - but they'll have written a "4" in permanent marker over the top of the "3" on the box.