Depends on your time management and organisational skills to be honest. I did 5 AS levels and completely regretted it as I simply didnt have enough time to spend on each subject since I struggled with a few of them. I got decent grades in the end but if I had the chance to do it again I probably wouldn't. Having said that I currently do 3 A2's and Further Maths AS which I'm coping with quite well. It's not quite the same as doing 4 A2 but I have the same workload, number of lessons and three more exams because of it so feel it can be used as a comparison! I quite like maths so I feel this is why I'm not struggling as much this year whereas last year I took graphics as an extra subject which was overwhelmingly time consuming and irrelevant for my chosen course. I took it because I enjoyed it and didn't realise how much work it would be so if that's your basis for doing psychology make sure you check the subjects structure before committing to it, how easy/hard you find it and also it's impact on your uni choice