So the standard for AS levels is generally to do 4, and the workload is pretty fine. I know people who have done 3, but nearly all of them had lower GCSEs than you, and I know several people who have done more than 4 and been fine. Technically the fewer you do the better your grades should be, but the workload for 4 is really not bad. At my school, doing 4 leaves you more than 6 hours a fortnight of free periods, which is plenty.