The able student who wants to do a degree which is maths related at a top tier uni, should be able to do 4 subjects,including FM without too much difficulty. People getting offers from top tier places, will expect to have an offer including A and probably A star in FM.
A student doing FM at GCSE, who is pretty much a dead cert for a 9 at GCSE maths and who is doing well at FM GCSE should be able to cope. Bearing in mind that the courses you’ve mentioned at uni are highly competitive and hard work, applicants need to be able to cope with workload and levels of stress too.
So, is your DD just a bit anxious and lacking in confidence that she ought to have but actually very able, or is she just a solid mathematician and student?
Askher teachers for advice. They will be honest. And as others say, starting with 4 and dropping FM is an option. A student who does that though, won’t be applying to top tier unis for maths related degrees, but then most won’t be capable of those degrees. No shame in that.
For the typical A Level student and for most not doing a maths related degree, doing 3 is generally better. If FM doesn’t feel a struggle and take up lots of time which detracts from other subjects, 4 is fine. For others, ending up with an A or 2 amongst their results means they would have been better with just 3 in many ways.
And for anyone considering 4 without the Maths/FM combo, advice is strongly to to do it. 4 absolutely won’t be needed and students stretch themselves too thin. It’s common for the candidate who could have got 3 x A star to just end up with one and a few As.