Maybe you should try asking the medicine subforum, you'll probably get more responses and there will be people can advise you about imperial specifically
But anyway I don't even think you have to do 4 A2s, I think most medicine courses give AAB offers plus you need as much time as possible for extra-curriculars/work experience/volunteering....Just about everyone applying has the grades, you need to be able to demonstrate that you'll be a good doctor , that you know what being a doctor entails, and that you have a life outside of studying..
Edit: plus I think there are courses that you don't need A2 biology for, but I might be wrong (I don't do A levels)