It’s always best to start with 4 AS and do 3 A-level 4 AS gives you a backup in case you change your mind or end up doing badly in one subject, if you did 3 and did a subject you didn’t like or did bad in, then what? You have to squeeze an a-level into one year or do an extra year.
Doing 4 A-levels is also alright but unis only want 3 most the time, sometimes having 4 can make you look stronger and if you’re doing something like further maths and it’s required they’ll usually ask for 4 a-levels