pathosL I think I'm also in a similar situation to you.
Currently about to start Year 13 having done Maths (Mechanics), chem, bio and phys last year.
I also considered a fifth A level but I think after doing 4 this year, while the workload during the year may not be too intense, the stress and pressure during exam time definitely is!
Edit: As mentioned earlier, it's even more pressure/stress considering there is no longer January exams.