My son has dropped Biology and is doing Chemistry, Maths and Physics. There are a few med schools which require Biology, but not that many. As Sanity said, QUB was still asking for an AS in a 4th subject when we were looking. If you know you are interested in any particular med schools, e-mail their Admissions people for advice - they are generally very friendly.
If you are doing BMAT, you will find it useful to have Y12 knowledge in all 4 of your subjects.
NO universities I have come across really mind whether medics have 3 or 4, including Oxbridge, but most insist that you get the grades for their standard offer, which will be for 3 A levels, so AABB is not as likely to get you in as AAA.