5DIII AF performance with f4 lenses ?

With f/4 lenses, the ONLY thing you don't have working are the diagonal crossed prisms along the middle (hence no double-cross-type AF points along the middle of the frame). But you still get cross-type points all over the frame. It's only at apertures of f/5.6 or smaller that you lose cross-type points at the periphery (but maintain cross-type points along the middle). So basically, at f/4, your AF is AT LEAST as good as the D4's and D800's.

And remember, the D4 and D800 don't even have double-cross-type AF points to begin with!