At GCSE, I got 9, 9, 8 in maths, physics and chemistry respectively, and now I'm just finishing my second year of maths, further maths, physics, and chemistry at A level. If you definitely want to pick four subjects (which I wouldn't advise), make sure you don't have many that require a lot of memory, that is, don't pick both history and chemistry. Physics is an easy A level if you're good enough to pick further maths, I don't know about economics though.
I would suggest picking: maths, further maths, and either history or eng lit
OR: maths, further maths, and any two of: physics, chemistry and economics
As I said, I don't know about the workload of economics, but history and literature will both require study of old texts and memory of past events, plus a huge load of essay work. Chemistry requires a lot of information to be remembered in an annoyingly specific way. Physics just revolves around learning to use the ~80 different formulae (most of which are just a=bc), which you get given in the exam anyway.
You also might want to reconsider picking further maths, there's some fun stuff in there, but it's rarely needed for uni (In my FM class of 8 people, only one actually needs it for their chosen course, pure mathematics).
I realised I've waffled a lot here, just try to find a single broad field you'd be interested in taking further (eg; engineering), and find out what unis want for entry into those courses.