As a lawyer, my observation about A level law is that it's a waste of an A level on a subject which can't be explored with much thoroughness within the confines of sixth form study. A student might benefit more as a person by studying, for example, English literature, or a language, or history, or a science.
Studying law at A level does not nothing to enhance a person's chances of becoming a lawyer later on. There is something to be said for the idea that law is a subject best studied at postgraduate level, as in the United States; but even if a student wishes to take an undergraduate law degree, I would suggest that taking another humanities subject, or a science subject, at A level could produce a more rounded person.
I add that taking four A levels confers no advantage when aiming to study most subjects at most universities, and may result in disadvantage if taking four A levels results in one or more lower grades amongst the four subjects, rather than three good grades.