Is taking 4 A levels worth it

I think it would be a good idea to take 4 levels initially. From the sound of it, it seems you might end up preferring to do more subjects and it may feel safer to take physics as well just in case. Considering you excel in all of the subjects you've picked and you prefer to have a bigger workload, I see no problem in you taking physics. As you said, if it does end up being too much, you can always decide to drop it later on, it's worth taking the chance by doing it now to make sure you don't miss out. Personally, when I dropped a subject it felt relieving to go from 4 to 3, although, since you do prefer having a challenge, you might find that you enjoy physics since it is a complicated and broad subject, it also does have a bit of a crossover with the mechanics section of maths A level too, so you may find it helpful.

The subjects I'm sure I want to take are biology, chemistry and maths. These are the subjects I've already confirmed with my college, but I'm also thinking about taking physics as a 4th. I really don't know what kind of career I want to have and some careers in physics have always intrigued me. I know for sure I want a science career but I'm not sure which science I want to take. Not taking physics will hugely limit the courses I can take at university which I would be interested in. I don't know what career I want and I don't want to decide right now and it feels as though not taking physics would be a decision not to pursue physics. For reference of my ability, I took 3 sets of mocks which I achieved grade 9s in for all 4 of these subjects. I'm quite afraid of burnout but personally I get burnout from boredom and lack of a challenge rather than too much work. I also figured I'd be able to drop one subject if it did turn out to be too much. I'd really appreciate some advice from anyone who has been in a similar situation