A-level maths teacher and 6th form tutor.
Lots of good advice above, if she was my tutee I’d encourage her to stick with 4 until end Y12 (with the caveat that she’d drop one quickly if grades in any of them started to suffer or she was stressed) and then definitely drop one at the start of Y13 (with her AS results to guide her and perhaps Uni visits and discussions with admission tutors).
Obviously I’d be on the internet and phone to the Unis she’s interested in to get their opinion, they hold the keys in this situation.
Good luck, there’s lots of tricky decisions to make around this time. I always try to remind my students that although it feels like these things are binary in a happy/not happy sense actually whether they are happy in later life has got very little to do with these decisions. Better to make the best decision you can then keep moving forward.