I think there will be some technical improvements when GT5 is released but nothing mind blowing.
As Polyphony gets better at coding for the PS3 (as all developers will) and get more out of the cell processor, there will probably be better AA, smoother frame rates, better lighting, and crisper textures. It's obviously not going to be a day to night difference but GT5 will probably look and run better.
And i think GT4 will end up on the PS3 4-5 years down the road, nearing the end of the PS3 life cycle.








