I'm not aware that "four A levels" is a requirement for ANY job
What happens to people who've only done 3 A levels - do they have to take a 4th one while they're at uni to remain competitive? What about people who took A levels before the A* was invented - do they have to retake all their A levels to get A*s in the current version of the exams so their CVs don't get thrown away?
Seriously, in your position I wouldn't do anything that would jeopardize my uni work, and if I did find myself with 'spare time' I'd be looking at taking up other activities to enhance my employability, not spending time retaking school exams.