GT1 and 2 were for PS1
GT3 and 4 were for PS2
GT5 is for PS3
Not much improved between 3 and 4 as far as physics, but there were small improvements. 3 and 4 were FAR ahead of 1 and 2 though.
I wouldn't say that 4 was not much of an improvement from 3 though. Do you know how much was added to 4 from 3?
GT3: 150 cars, 34 tracks, A-spec only
GT4: 700 cars, 51 tracks, A-spec & B-spec
That's a pretty big improvement IMO, not to mention the slight graphics improvement and slight physics improvement.
In any case, GT4 was in no way disappointing.
Jeff