From the Cambridge website:
Our typical offers are based on students taking three A Levels together in Year 13, and most Cambridge applicants are studying three or four A Level subjects in Years 12 and 13. This is usually sufficient to show breadth of interests and ability to manage a range of differing academic tasks. We’d rather applicants develop broader and deeper knowledge of the subjects most relevant/closest to their chosen course than accumulate additional A Levels.
Applicants taking four subjects won't normally be at an advantage compared with those taking three, although competitive applicants for STEM courses often have Further Mathematics as a fourth subject.