I think because of the high drag the Mig-21 experiences in a tight turn he'd better get a shot off quickly, especially against a slat-wing F-4.
Because the slat-wing F-4's have pretty much the same turn radius as a Mig-21, and since the F-4 has a power advantage the Phantom is going to be able to maintain the turn longer.
And of course the Mig-21 is in a fuel emergency state shortly after wheels in the well he'd better stay close to his base.
Having said that, our hypothetical F-4 pilot probably refused to get into a level turning contest in the first place.