Some admissions statistics, according to Oxford. These are from 2013 and below, but should give an indication of how Oxbridge give offers:
2013, Oxford Admissions, page 26:
https://gazette.web.ox.ac.uk/files/admissions-2013pdf"Candidates may have taken more than 3 A-levels but table shows only the best 3 A-level"
Candidates with A*A*A* or above had a 44.6% success rate of gaining admission.
Candidates with A*A*A had a 30.3% success rate of gaining admission.
Candidates with A*AA had a 21.9% success rate of gaining admission.
Oxford mostly focuses on the grades of your top 3 A-levels. As you can see, candidates with 3 A*s had a significantly higher chance than candidates with A*A*A. Presumably, if you achieved A*A*AA at A-level, this might be perceived as a weaker candidate, compared to someone with A*A*A*.
Unless you are expected to get the grades A*A*A*A or A*A*A*A*, then no, it wouldn't be worth studying 4 A-levels.
The only time it would be worth it, is if you got at least A*A*A* in your 3 A-levels. In that case, a 4th A-level would unlikely be that much of a benefit anyway.
Based on this, I would assume it's best to focus on getting A*A*A* on 3 A-levels, than getting A*A*AA on 4 A-levels.