If you did AQA for physics GCSE before, then its better you do AQA for As-level. Although your taking 4 A-levels currently, ideally its better you take it in a gap year as you will have more time to focus individually on that one subject and just focus on your other A-levels your currently doing. If you really put your mind in to it, then you could manage and do your AS but that will require you to do constant work everyday, considering that physics is probably the hardest A-level, not sure about the AS part though.