I'm doing four A levels currently, but mine are all in humanities subjects and the fourth I am teaching myself. So, I'm not sure how much help I can be. The only reason I think its not so hard, is because of the circumstances (humanities, self-teaching etc) but I know of others who have dropped down to three, particularly with regards to science subjects, having found it too hard.
If you REALLY want to do it, then its possible, but it can be a lot of hard work with certain combinations of subjects and unis do only ask for three grades. That said, Im glad I have four grades to get my ABB offers from, particularly since in one subject Im doing shockingly bad so far!