It wasn't quite as simple as that, wiggy. I've just finished reading Boyd's book (which I'd highly recommend, BTW), and, as MTOW said, the MiG 21, properly handled, (as it usually was by the North Vietnamese), was superior to the F4 in just about any regime or circumstance you'd care to name.
From Boyd's book:
If there was a turning point, a time when even the most jingoistic Air Force general at last understood that Communist forces could build fighter aircraft superior to anything that America put in the air, it was Vietnam in 1967, the worst year of the war for the Air Force. It finally sank in that, as Boyd had said for years, the Air Force had no true air-to-air fighter. It is said that combat is the ultimate and unkindest judge of fighter aircraft. That was certainly true in Vietnam. The long-boasted-about ten-to-one exchange ratio from Korea sank close to parity in North Vietnam; at one time it even favored the North Vietnamese. When the war finally ended, one Air Force pilot would be an ace. North Vietnam would have sixteen.
At the time, the North Vietnamese Air Force was shredding the ranks of F-105 drivers. So many F-105s were shot down along a mountain range near Hanoi that the pilots called it "Thud Ridge."