I would definitely recommend doing 4 AS Levels - not because universities prefer you to have the extra AS, but because it gives you greater flexibility to drop something you're not enjoying. A good number of my friends only took 3 subjects at AS Level, and by the end of year 12 absolutely hated at least one of them, but had no choice but to continue with it because they had no alternative subject to fall back on. The workload isn't that bad, honestly.
Also, did you mention having to go to another school to study a certain subject? I had to do this to study Economics in year 12, and I'm SO glad I did. You widen your group of friends, gain confidence... it's a win-win, really.