Im not sure why she would need to drop one - about 90% of my yeargroup did 4 subjects at A Level. It's not uncommon, and there isn't a workload increase from yr12 to yr13
How long ago was this SomeoneElseEntirelyNow? Since the A level reforms most A level students take 3 subjects, unless the fourth one is further maths or they really are exceptional students.
I did 6 AS levels and 5 A levels (Maths, French, English, General Studies and Spanish where I did AS and A level in one year).
A level reforms means that all subjects are linear SuperrHann. Basically you sat the equivalent of 6 half A levels at the end of year 12, and 5 half A levels plus a whole A level at the end of year 13. Which is an excellent achievement, but not the same as sitting 6 whole A levels at the end of year 13.
OP if your daughter is achieving the highest grades with 4 subjects there is no reason why she should drop a subject. Medical schools only ask for three subjects though. I would take the advice from people who took A levels when AS levels were still being taken with a pinch of salt because since the reforms all A levels are linear. They sit the entire contents of two years work at the end of year 13, not half at the end of year 12 and half at the end of year 13.
Interestingly, DD is just about to complete her first year of a biomedical science degree. She has several friends studying medicine, and she has said that in the first year her course has been more demanding and gone into more depth per topic than that of the medical students.