The problem is in the "core" of the game...it's a sort of compense for human driver, since, if You increase too much skill value of drivers ( over 16500 is hard to compete, and over 17000 is impossible ) in perf texts, or if You increase too much grip driver in magic files ( for tracks ), then CPU drivers become unbeatable, at ace level
In GP4, infact, there are several physics and mechanical features balanced between human and CPU drivers, and this balance is not perfect.
Some things are better for human driver ( as for example, the starting grip ), some other are better and costant for CPU drivers ( brake power and space, trajectory, and so on ).
You can try to adjust the Physics file, or the Torque value in GP4exe, or even the perfomances text and magic files, but, believe me, You won't found a solution to this defect.
The only right thing is to make some good compromises.
When You set in a realistic way performances, after the start and the first corner ( where You can get some positions ), then CPu drivers are difficult to pass, and if their grip ( in magic files ) is too high, CPU drivers will have ever a shorter brake distance, and more acceleration...although the Physics file for brake and tyre grip affect both human and CPu drivers.
Briefly: what You are saying is one of the well known defect of the game ( like for example the unique main cockpit, or the unique car sound, and even the unique Physics file...), but in the complex GP4 has still many good features and advantages respect other F1 simulators.
Surely, if I could make another new F1 simulator-game, this is a characteristic that I personally should improve.