GCSE Chemistry was a simplification, A Level Chemistry is a really big jump but with a lot of work in your own time, it is ok.
4 A Levels is very time consuming and not really necessary for uni, they only want 3 A Levels unless you’re aiming for Oxbridge. Do the subjects that you love. I did AS maths and it was pretty rough for me. It’s a useful qualification but unless you’re super passionate about maths, don’t do it.
Ultimately it’s up to you