As they are all quite heavy subjects in terms of essays and workload I'd advise you against it, I only took essay subjects at AS but they took up much more time than my 'sciences'. I know someone who took 5 who seriously struggled and it affected her grades quite substantially especially as she had resits on top of her other exams but I also have a friend who is the most intelligent person I know and seems to cope with 5 reasonably well. Obviously it's your decision but unless you're trying to keep your options open it's a good idea to stick with 4.