I only took three a levels and these were purely based on my personal interests rather than which career I wanted to pursue so I think it is really good that you are already thinking about why you want to choose certain subjects.
I knew people in sixth form who took four a levels so it is possible but there is always the option to drop one if the pressure becomes too much or you lose interest in one of the subjects.
Have you an advisor or a teacher (in those areas) you could email and ask for their advice?