Firstly, in your case I would say to choose only three A levels. Don't unnecessarily add to the workload because you'll regret it two years down the line.
Secondly, the maths in chemistry isn't too difficult conceptually, but it is in every nook and cranny of the subject. The majority of the maths is algebra related, and a lot of it doesn't really go beyond high level GCSE content. I'm sure with practice you'll become comfortable with it.