I'm gonna carry the case for doing 4! To be honest, it is a lot of work, but i have found that people in my school that do three are as stressed if not more than me, when you drop down to three you say your going to use the time to improve your 3, but you end up bumming around. If you don't believe me, one of my best friends dropped and last year he was a workaholic, and this year he just CBA!
Also, 4 a-levels offers you insurance. I am currently doing 4 and am on A's (all above 260 ) for all of them, and i ahev to satisfy a BBB/ABC offer, so there is a lot of scope for error. With a bit of luck, i could feasibly get ACCC and get into uni (with two C's equaling a B, altho it would take sum pleasing's and thank you's). There was a case with a guy last year for law at Nottingham, he needed AAB and got ABBB and got in as the two B's equalled an A. It can save you if one a-level slips.