To be honest, if you're already doing 3 and not particularly interested in a career with Physics or Economics, it doesn't really matter. You'll NEED physics if you want to take a physics or engineering course at uni, but if not there's no particular reason to take it as a 4th.
The same pretty much goes for economics, except you could probably get away with just having maths if you want to do something economics-related at uni or in the future.
You could always start with Physics, and if you find it too difficult or not worth it, your sixth form would probably let you swap to economics in the first few months. That's pretty much the only way to really find out if you'll handle it well.