I only have one idea which is kinda ugly...
but you could change the Magic Data of every track to a lower fuel consumption. I can't give you any maths for that but I am kinda sure this would work because AFAIK GP4-AI will drive until they have fuel for less than 2.0 laps loaded. The next lap they will pit. Altering the fuel consumption will still have them start with 4.0 laps worth of fuel but that will wear down slower. Obvious drawback: AI will have way too much fuel during the race. I'm not sure though what happens if you save after the qualifying, close GP4, change the MD to the default value and reload the save game.
Here's hoping that there is a better idea :D