BUT the M-66-4 has the endurance package introduced with the M-66-3 and is a more desirable gun for the shooter.
The difference in price also probably reflects greed and what the seller thinks he can get.
I wouldn't pay over $500 for any M-66, and I'd insist on at least NRA Excellent condition, although I'm not too adamant about a box. However, I am prepared to walk many miles at gun shows until I achieve that goal.
It is ALWAYS best to actually inspect a gun before purchase.