It really doesn't matter what you do a 4th in, med schools only care that you have biology, chemistry and one other to full A-level (and maybe a 4th to AS).
Personally I found psych to be extremely useful for the health psych content of the first 3 years of my medicine degree, but do whichever you will enjoy most & do best in.
But bear in mind that most med schools will not consider further maths to be a separate subject from maths, so if you are planning to apply somewhere that requires a 4th subject at AS then you should take psych.