It depends on what you want to apply for and where. My firm offer included getting a C in a fourth AS (which I already had, but they still included it) and I know that was fairly standard for the course and uni. Have a look a the prospectus of places you want to apply and see what they say about the fourth subject. You won't be hindered for not doing four A2s as three is the standard, but if you need the fourth AS to be a certain grade, it might be worth resitting some of it. And of course, don't write it off until you get your actual grades.