Most places won't give four grade offers, but I think that oxbridge will sometimes ask for AAAA if they know you're taking four. Personally, I'd go for doing three and doing really well in them rather than stretching yourself between four and maybe not doing as well.
I was really concerned because at AS I got aaac. I was only concerned because I thought that the c really showed me up because I'd obviously done well in my other subjects. I still got five offers though, so clearly my inadequacies at Biology didn't bother the English admissions tutors as much as they bothered me.