This comes up quite often. The view that further maths A level isn't quite held in the same regard as others may stem from the Cambridge response 'Number of A level subjects studied' found here where they state that:
"Further Maths is thus a particular case which should be considered separately from others. In many ways it is effectively not a fourth, distinct A level, but grows out of work done for A level Maths. The combination of Maths and Further Maths is not, for those strong in the subject, as heavy a burden as combinations of Maths with any other A level."
or it may be that for some degree courses only one of maths and further maths will be considered (this is often for medicine courses, but is becoming rarer - Edinburgh is a current example)
Cambridge also state that three academic/traditional and relevant subjects at A level are usually quite sufficient, and more are not often encouraged especially for arts subjects but ...
"For those who want to apply for mathematical and science subjects we are aware that the academic value of taking four A levels is clearer. We certainly would never wish to put off an applicant offering, say, three sciences plus maths for our science-based courses, and would very actively encourage all applicants in Physical Natural Sciences, Computer Science, Engineering, Chemical Engineering and Mathematics to take Further Maths if available (and, if not, to see how else they might improve and develop their mathematical knowledge and skills according to the advice offered on both the University of Cambridge Engineering and Mathematics Faculties? websites)."
So, if you want to study Nat Sci, comp sci, engineering or maths at Cambridge, and you have the ability to do 4 without grades slipping and your school supports it, then it's probably a good idea.