You don't need 4 by any means, especially since there are some schools that don't give students the option of doing 4 anyway. I'd imagine the trend would simply be because Oxbridge-calibre students are more likely to be capable of doing, and want to do, more subjects. You're not really at any disadvantage by only doing 3, as (1) taking on extra workload is only likely to bring your overall grades down anyway, and (2) if you take 4 A-levels some colleges might give you an offer based on all 4 rather than just 3.