I'd take a look around universities first. Certain courses may require the 4th AS, although it's fairly unusual for this to happen. The only universities I know of that ask for it are Oxford, Cambridge, UCL, Warwick and Nottingham.
If it doesn't say you'll need a 4th, don't worry about it! The main purpose of an additional AS is so that you've got something to fall back on if you don't do too well in another subject (hence why 4 subjects in first year, usually 3 in second). However, if you really enjoy 3 subjects in particular and you know you'll do well in them, I'd say do it.
From personal experience, 3 AS subjects was one of the best decisions I've made. I got my results a week ago, my two A grades were 88% overall for Geography and 94% overall in Politics, and I don't think I would've been able to do that if I'd had the pressures of a 4th subject.
You can also take up an additional AS in your second year, once you're into the swing of A Levels.