Doing a 4th a level now in year 12 for medicine

I would say you are better off sticking with the subjects you are doing now, as with UCAT +/- BMAT and interview prep, a 4th A level in a year would risk dropping achievements that are critical.
If you then achieve A*AA in the subjects you are already studying (for most London med schools) and an excellent UCAT and BMAT, you would still have the option to take a gap year and sit chemistry in this, knowing you have experience of entrance tests and 3 good A levels in the bag. Most med schools would be happy with this, if you had A*AA already achieved, and then got the required chem grade, as well.
Of course, if you had offers from your first application round outside of London, most of us would say you would be silly not to take them