Given the same generation fighters, flown by *competent pilots, the determining factor in an air-to-air engagement is something known as the 'unseen fighter'.
Example: Two F-4s are engaged with two Mig-21s. If another Mig-21 were to join the fight, that side would likely win. Conversely, if an F-4 were to sneak into the fight, that side would win.
*The pilots dont have to be the best, just competent to fly and fight in the plane.
Source: The Air War over Vietnam.