If you have your mind set on the top universities then UCAS points won't matter, they use grades and not UCAS points to give offers. Uni's make offers based on 3 A Level grades most of the time.
Most ppl will drop their fourth option after year12 and anyway they recommend a fourth option just I case you don't have three good grades (it's just in case you fail in one alevel - nothing to be concerned about though as long as you work hard) I know quite a few clever or not who will be either not doing a fourth subject or will intentionally pick to drop it. Unis only want three subjects so do three subjects if you want to