I’m guessing you are in either NI or Wales considering you are talking about AS levels? Not sure if Wales have numeric or alphabetic grades at GCSE, so are you maybe in NI?
If so - taking 4 subjects in NI for AS is much more common than anywhere else, however it’s still not the majority who do it. (And yes - here, AS still counts as 40% of the final grade) Some grammars push for everyone to take 4 at the start, but many (most) have a points requirement to be allowed to take 4. (Often it’s something like 12 points to get into 6th form, and then 20/22/24 points to be allowed to do 4 subjects) The thinking is that if you aren’t achieving highly at GCSE then you are going to find 4 subjects incredibly hard work. Your DS has 31 points with those grades so clearly very capable, but it really depends on whether he needs a 4th or not.
Pros of doing a 4th are that if the pupil is struggling to decide between subjects, and/or choosing a new subject which they have never done before, it gives them some scope for choice further down the road by dropping one. It is actually pretty common to have pupils drop their 4th by October, or Christmas (often around mocks time) when they realise it’s just too much and they want more study/free periods in their timetable. Alternatively, they keep all 4 for AS and then decide what to do after results the following August.
In terms of university, in the UK the offers will always be based on 3 subjects so a 4th is not necessary. If considering ROI however, a 4th (even just at AS level) is pretty much essential because of the points requirements. (There is talk of this changing, but not sure how quickly it will happen)
The cons are that it is A LOT of work. I cannot stress this enough - the workload for 4 is huge, so it really does take a capable, but also very committed and motivated pupil to do well with 4.