I do 4 A-Levels and an EPQ and its really not that bad. EPQ isn't as much work as people make it out to be and really isn't equivalent to an AS level but it can be fun to do, which is why I did it. I was wanting to learn about this maths topic anyway so I just used my EPQ as an excuse to do that and get a grade out of it. 4 A-Levels is manageable but I do maths and FM so it's only really 3.5. 4 completely different ones may be a bit more challenging but nothing impossible if you genuinely enjoy all 4. Btw for subjects like CS, engineering, natural sciences and econ at cambridge you are really advised to do 4 and the majority of ppl that get in do 4. There are exceptions of course. Im applying for maths this year and although it certainly isn't required to do 4 for maths, I enjoy all my subjects and my EPQ and am doing well in all so why not take them all.