Here's the info that I have managed to collect from around the web. It appears the game will pretty much be a direct port of the PS2 version - only handheld!
Less Cars and Tracks
There will be a few less cars and tracks available.
However there will be extra ones that aren't in the PS2 version to compensate.
Multiplayer
There will be a multiplayer mode in the form of the PSP wireless support. So 4+ people with a PSP and GT4 Mobile in the same vicinity will be able to play against each other. No news as to whether internet play will be supported via the PSPs' yet to released internet gaming architecture. Needless to say that would be very cool.
Graphics
Graphics will not be quite on par with the PS2 version, a little less scenery, a few less polys. Personally I don't think this should be very noticable considering the PSP obviously has quite a small screen compared to a regular TV. Also, only 4 cars will race and not 6 as in the PS2 version.
Photomode
Photomode will be available in the PSP as it is on the PS2. The images will be in a higher resolution than normal as with the PS2 version. (I don't know what this higher resolution will be yet though). You'll also be able to save your pictures on a memory stick, which can then be copied to your PC.
There are some screen shots and a couple of video clips available at IGN and Gamespot . Most of the info above I gleaned from the PSXExtreme website.
Less Cars and Tracks
There will be a few less cars and tracks available.
Multiplayer
There will be a multiplayer mode in the form of the PSP wireless support. So 4+ people with a PSP and GT4 Mobile in the same vicinity will be able to play against each other. No news as to whether internet play will be supported via the PSPs' yet to released internet gaming architecture. Needless to say that would be very cool.
Graphics
Graphics will not be quite on par with the PS2 version, a little less scenery, a few less polys. Personally I don't think this should be very noticable considering the PSP obviously has quite a small screen compared to a regular TV. Also, only 4 cars will race and not 6 as in the PS2 version.
Photomode
Photomode will be available in the PSP as it is on the PS2. The images will be in a higher resolution than normal as with the PS2 version. (I don't know what this higher resolution will be yet though). You'll also be able to save your pictures on a memory stick, which can then be copied to your PC.
There are some screen shots and a couple of video clips available at IGN and Gamespot . Most of the info above I gleaned from the PSXExtreme website.