You are already ahead of the game by taking four AP courses. I assume AP Math is also one of the courses you are already taking. If not, I encourage you to take AP Math. I would also add AP Physics as long as you think you are ready to handle the course load. AP Biology wouldn't hurt either, but it is not as important as the others to Chemical Engineering. However, also realize that you'll have time to cover these subjects in college, so don't worry too much if you decide to wait until you enter college. Best of luck.