The discussion of UCAS points is misleading. Neither university uses UCAS points in decisions. The claim that getting four A*s gives you a 64% chance of getting in is nonesense and ignores how Oxbridge admissions work. A level grades are not a major factor in decisions. They don't really care how many A*s you have, as long as you're above the threshold. Oxford Physics primarily uses GCSEs, the admission test, and interview scores in decisions, not A levels - see the department website:
https://www2.physics.ox.ac.uk/study-here/undergraduates/applications/admissions-procedures-for-physics"The requirement to take 4 A levels is not stated on their website" - yep, because it isn't a requirement.
Neither university says that 4 a levels makes for a more competitive application. For example, Oxford says:
"Three A-levels is absolutely sufficient to make a competitive application, and offers will normally only be made for three A-levels regardless of how many you are taking...
If you choose to take more than three A-levels, this will certainly demonstrate an ability to handle the workload we place on Oxford students. However, be sure that you have plenty of time to read around your chosen subject beyond your school or college work. You will certainly be more appealing than a candidate who may have taken more subjects, but who is not able to discuss their interests with any enthusiasm or in any depth...
We also advise candidates not to spread themselves too thinly across too many subjects, where they may risk dropping a grade or two in their results. Our offers are made on the basis of exam grades rather than UCAS Tariff points, so three A*/A grades*would be sufficient to meet most conditional offers, but two A*s and two Bs would not."
I for one think there may be advantages to doing four a levels / an EPQ. It may indirectly help you in other areas of the application (e.g. improve subject knowledge or problem solving skills) But, as made very clear above, doing only 3 does not disadvantage you. The fact that many physics students have four a levels does not mean it is prerequisite for admission (i.e. correlation does not mean causation)