History and Business Studies are not going to help with medicine at university. Why take an extra useless A-level when you could use that time to focus on the much more important 3 A-levels you need to do well in?
Regardless, you seem to be fairly determined to do 4. I guess I'm biased, but do History. It is likely to be more valuable than a Business Studies A-level, it can improve both your writing and analytical skills quickly, and it'll help you out when it comes to playing trivial pursuit.
A-level is not just harder than GCSE - it's a completely different world to it. The way in which you study and apply your knowledge is much different in A-level. You will need those free periods - don't replace them with an A-level you're probably not going to use.