4 tracks, Tsubuka, Laguna, New York & Nurburgring are in both games. The main differences visually is that the graphics in Forza are bigger, brighter & more colorful. There is also alot going on and the tracks just look 'busier'. In GT4 the graphics though more realistic, are blander and grayer and just seems to be dead boring to look at. I have watched a dozen replays in Forza, only watched 1 in GT4.
I also noticed that the tracks are wider and the cars appear larger on the track. The dipper at Laguna is fun to drive on and you can actually 'feel' the dip, but in GT4, the dipper seems small and insignificant & you can easily drive over it.
If I had a choice in which version of the tracks to visit, I know which one I'd choose!!
FORZA FORZA FORZA

I also noticed that the tracks are wider and the cars appear larger on the track. The dipper at Laguna is fun to drive on and you can actually 'feel' the dip, but in GT4, the dipper seems small and insignificant & you can easily drive over it.
If I had a choice in which version of the tracks to visit, I know which one I'd choose!!
FORZA FORZA FORZA