Doing 4 AS levels is totally fine. The workload is of course a bit higher than doing 3 but if you're good at your subjects and have some idea of time management you'll be totally fine.
Once it gets to the second year it does step up a gear. You'll need to be fairly good at all your subjects in the first place and also willing to put in 40-45 hour weeks To put it in context that's using all of your free periods, plus 2 more hours on a school day and 5 hours each on Saturday and Sunday so really not so bad. But by this point there's no way you'll be doing a part time job. And you'll probably resent all your friends who are doing 3 a levels and still complain about not having enough frees.
So I think there's no harm with starting with 4 and, as time goes on, decide if you think you ought to drop one.
If you decide you're going to drop one after AS level you might be able to ask your school to put you in for the AS level exams even if they don't do it as standard. That way you'll still have a qualification from your year's work.