Most people who took 4 have dropped one already, or are going to drop it at AS. It would be good to choose another one just in case you hate one of the one's you've already chose.
i was told to take four, but i still don't know what i want to do so i put down 5 :L but even if you do one at AS it still gives you some ucas points i think. Correct me if i am wrong.
Pick a fourth AS, it's the done thing. If you dislike one particularly, then drop it for A2. Again, that's the done thing for most people.
In terms of subject choices, it all depends on what course you're going for and what kind of unis you're aiming for. If you don't make these clear, then no-one can help you. If you just want a subject you enjoy, only you can decide that.