Hi, I'm in my second year of doing 4 now and trust me, it's a bit of a waste of time doing 4. Most universities don't take it into account, but Oxbridge might depending if it's relevant to the course or not. The only time I'd recommend doing 4 is if your fourth is further maths and you want to apply to do something around maths/ physics/ engineering etc.
I think a lot of colleges/ sixth forms are moving to linear now, so doing an AS is no longer an option there, but I'd check with the places you're applying to. Although I'm not sure how much AS's help in terms of uni applications nowadays, but my guess is not a lot.
Hope that helps!