All of my friends that wanted to do something mathematical at a top uni and did 4 subjects picked FM, Maths, Physics and Chemistry and all of them achieved A*A*A*A (all of their As were in chemistry).
I would suggest doing only 3 but if you want to do 4 then just pick the one that you think will be the easiest (often the one you enjoy the most and a bit less work (maybe has a coursework aspect)).
Maybe look at the specifications online and watch a couple of yt videos on topics and see which one you are most interested in.