In the ancient past I spent about 1000 non-combat hours displacing valuable fuel in the back of F-4C and F-4E/G airplanes.
I don't think any sane F-4 pilot would try to get into a low speed turning contest with a Mig-21, he would try to use the double ugly's power advantage to gain the vertical while encouraging his Mig-21 adversary to bleed off his energy by turning hard with his draggy delta wing.
My main thought is this: If you mission is to defend your air base from attacks by F-105s while under GCI control then the Mig-21 is well suited to that mission.
If you want to project airpower and operate with minimal outside assistance then the F-4, especially the F-4E and later models would be far better choices.
History shows in a direct encounter the F-4 will usually destroy the Mig-21 in combat.