Universities aren't interested in your AS levels, but more your A-levels as a whole. Traditionally, most people apply to university taking or having taken three A2 subjects. Not a lot of courses, if any, ask for four A2s.
Having said that however, there isn't a reason why anyone with prospects for going to university couldn't handle four AS subjects. While it isn't essential for you to do four subjects, there isn't any reason why you shouldn't.