You say on your UCAS form you're taking 4 A-levels. You then get a 3 grade conditional offer. You ring to tell up the uni you've changed to 3 (and have a legitimate reason for doing so)
Chances they'll throw out your offer?
My friend applied to Cam last year and her 3A offer specified the 3 subjects, so she decided to drop the 4th as it couldn't be included and may have affected her final grades - no problem. If the offer's made for 3 subjects that's all you need to get. Say for example you took 4 subjects and got 3As and a U you would have in effect dropped the 4th without telling the uni and the offer on 3 would still be valid.
You don't need to have a legit reason - just ring the uni say you want to drop x subject and is this ok? They most likely will say yes (as long as it is not a specified subject)