A levels are a long old slog to be honest. If you’re going to do 4 then pick an “easy” 4th one. I would personally suggest that you study Maths, Computer Science, and Physics. You can do Economics or Further maths as an AS level if you want, or depending on how you find it in year 12, you might decide to continue to your final year with 4 A levels. Also music has a hell of a lot of coursework from what I’ve heard as well so keep that in mind, although if you’ve achieved grade 8 then it shouldn’t be too difficult but you might just find it very time consuming. If I was you though I would go for Maths, Comp Sci, and Physics.