Adding stage 4 turbo just makes the car harder to drive.
It is easier to set the suspension up to handle only the stage 2 turbo, (you should be able to win those races you mentioned with that much power), than trying to control a 900 hp monster.
Also remember that with 900 hp you will have to brake a lot earlier than with 600 hp, so usually no set up can help your car corner when you brake to late and carry too much speed into the turn..