I was in the same situation as you, and dropped my fourth AS two months before my exams. To be honest, I wish I had just kept at it, although I came out with all As in my other subjects, it's added stress doing an EPQ now with UCAS applications etc. The EPQ can be considered as an equivalent to a fourth AS subject. It's up to you, but I would advise to stick at it, it will leave you in a better position for next year.