3 Vs 4 A-LevelsWatch this thread
you don't really need 4 a-levels for anything, but in some cases (eg. if you're taking a very generalist degree like natural sciences) it makes sense, especially if one of them is further maths. The vast majority of university offers would be made on 3 A-levels and 4 doesn't give you much of an advantage, but it can provide a fallback and insurance if you don't like one of your A-levels. (if you're unsure about your options. it's sensible to start with 4, if your school allows you, then drop down to 3 when you have a better idea of how the subjects are at A-level. If you are extremely sure then you might want to just start with 3)
The only exception to this is those needing to do Maths and FM for a STEM degree.
You risk all your grades by taking 4 - which is daft when if don't need to.
Remember - AAA will always look better than ABBB, and those lost grades could cost you a Uni place.
I am a current 3rd year at the uni of Nottingham.
I studied 4 A levels and no uni I applied to said they would change any offer or count the 4th. I ended up sticking with 4 as I did sciences and then a language as the 4th so enjoyed the variety and really wanted to learn the language however if you are doing one to help with uni sadly from my experience it is not worth it.
I hope this helps,