It's definitely fine to start with 4 A levels if you're interested in all of them - you can always drop one part way through Y12 if you find it's too much work. Do remember that if you are struggling at any point, A*A*A is much better than A*AAB (and no unis require more than 3 A levels) so if you find one subject is dragging you down then there's no need to keep it.
Also maths isn't a requirement for medicine except for Cambridge where you generally need at least 3 science/maths A levels. If you want to take it out of your own interest that's fine.