That's entirely up to you, but generally, people drop to 3 at A2 (2nd year) if they do 4 at AS. Just make sure you enjoy all of your subjects.
Maybe take 4 and drop one later on if you begin to realise you hate one?
Also, I don't recommend having a job during sixth form, particularly after January. I've had lots of friends get low grades at AS and improve loads at A2, possibly because having a job dropped their motivation to revise.