I know quite a few people who did 4 A levels, mostly including maths and further maths, and they seemed to manage fine but I'd personally say if you do 3, if you're aiming for A*A*AA with 4, you could get all A*s with 3, which would look more impressive, due to being able to give more attention to each subject but I can't tell you directly as I only did 3, and found that stressful enough, and unlike it sounds for you, I had to work reall hard to get average grades at GCSE and spent 99% of my day at my desk to get high UCAS, and hopefully actual, grades so you may be fine with 4. Also agree with the above as most people I know started with 4 then dropped one that was either too difficult, not enjoyable or in a different style from how they were expecting