There is information on this in the FAQ (Slipstream and Performance files).
Adjusting the physics is a fine art, it requires experience with GP4. There is a lot of information within a physics file and understanding the numbers takes a lot of time. You can adjust the AI of the other cars in-game by increasing the difficulty level and or changing the performance file, once again, you need to have an understanding of the numbers.
Send a pm to phantaman, he may just have something that would suit you.
[
www.mediafire.com] Some say you should click it, you know you want to.
[
www.gp4central.com] <----GP4 Central