Tell me what is your top speed? I would like to know where is the best line to do it. My is 313.xxx mph with MERCEDES-BENZ CLK-GTR. Mabie there is some way to make it up to 4 mph?
I personally doubt that 400mph is feasible - I'm sure that after a point air resistance must eventually form a barrier so your car is physically unable to go past a certain speed.
This is posted in the wrong place. It's not a race report.
If you check in the GT4 tuning & settings forum, you will find a thread titled "Three Hundred Miles Per Hour". Use that to ask your questions in/get some tips.