Many medical schools are no longer looking at AS Levels for this year's applicants so it definitely won't put you at a disadvantage. A lot of them aren't bothered about essay subjects either, they'd rather you had the sciences/maths (most require at least 2) and if your essay subject was at AS Level it wouldn't be considered anyway.
Physics AS is pretty interesting and crosses over with your other subjects quite a lot though so I'd go for that if you wanted to do 4.