A lot of schools like you to start 4 and drop one around Christmas or after AS once you have decided which you like best, are better at and is taught best in your school/college.
For medicine some places require 3 A levels + an AS or EPQ.
The only real reason you might take 4 all the way to Y13 is to improve your university choices if you are taking Further Maths, as it can be a bit limiting if you are doing Maths, FM and 1 other.
Obviously needs to be your choice, but if you want to persuade your parents, get them to "Ask a Cambridge Admissions Tutor" on here and they will be told that you only need 3 even there!