As someone who is doing Physics at uni and did Further Maths at A Level, do Further Maths. It'll be suffering but I promise you'll thank me later when first year of uni becomes a breeze because you've already covered like 80% of the maths you need to learn for it. Everyone I know who didn't do Further Maths struggled a lot with learning all the new mathematical content on top of the physics stuff, and were even recommended to take different maths modules by the uni to accommodate for their lack of Further Maths.
Oh and unis really don't care about your 4th A Level either way, so don't worry about that.