I agree with what was said about Further Maths. I loved it and am now a medic at Cambridge, but if I remember correctly, I couldn't use it as one of my As in my offer. I think for other medical courses they would prefer you to have done a more essay-based subject for your fourth choice, but I doubt Cambridge are too bothered either way as the maths does come in helpful. It's a bit of a gamble at the end of the day, I wish I could tell you what the right choice was!
EDIT: I actually did FM instead of biology - now, that IS something I wouldn't advise! It isn't a problem once you've survived the first term, but I'm convinced it must lower your chances of getting in (although all admissions tutors are different).