They only ask for 3, but I don't believe they wouldn't see 4 A's as being more of an achievement than 3A's, just because they ONLY ask for AAA usually.
That being said, I don't think they would see AABB as better than AAA, or AAAB for that matter. Sure, if you enjoy it and don't think you'll struggle at all, then by all means go for it. If you can get the AAAAA, then there's no reason not to.
As for Imperial asking students doing 4 A levels to get 4As, that's untrue; like most unis that specify 4 grades, it's AAAC.