But if you do 4 a levels then the uni should give you an offer consisting of 4 grades - unless of course they use the UCAS tariff system. Not too familiar with the points based system though, sorry!
They only expect 3. Only the top unis have much mention of 4, but then again it is very competitive. Three good ones is always better than 4 mediocre ones.
I took a lot of A levels (see profile) - if you want to do more because you enjoy the subjects, are good at them, and won't do badly in them as a result of having a higher workload, I think it is fine to do so. I enjoy studying/learning, so didn't think of it as doing more work than "necessary".
6 of them were directly useful to my degree (maths, further maths, additional further maths, biology, chemistry and physics), chemical engineering. Although it would have been fine to learn the required content at uni, having done these A levels probably gave me an advantage. Self-teaching maths was also helpful for gaining study skills required at uni.
The rest (Music, Russian and French) I did primarily as I enjoyed them. Music is useful knowledge for the extracurricular music I do, and being able to speak/understand other languages is useful, and broadens cultural understanding.
At the time, I did AS critical thinking for the sake of it, but in hindsight, at times it has been useful to be able to identify parts of an argument with ease and analyse an argument. I think our school taught it, besides from being part of the AQA Bacc, as it would be useful for verbal reasoning admissions tests. I did an EPQ as I thought I might as well. I guess it was good presentation practice.