I did 3 a levels and that's a crap load of work already so I personally would have never chosen to do 4 as 3 was already a lot of work. It was difficult to contain it all especially in the second year where the amount of information is even more than double the AS year.
However, from what I've seen/hear on here and from people I know personally who did 4 a levels, almost all of them dropped one of the 4 in the A2 year. Usually the one they got the worst grade in. They all dropped it as it brought their AS grades down as they had to focus on 4 subjects instead of 3 and believe me, there's a humongous difference between doing 3 a levels and 4.
It's not worth doing 4 either as if you want to go to uni, then you only need 3. The uni will disregard one of the a levels if you chose to do 4 so it won't even be considered.
When it comes to physics or maths. I would say don't look at how good you could do at either one but at whether you'd actually enjoy doing the subject. If you know you won't enjoy it then don't bother choosing it. Think of both in terms of how much you'd like to do the two and then pick one or the other. But you have to remember that physics & maths at a level are one of the toughest a levels out there