The broadcast industry works on drop frame.
I used to have a paper on it but can no longer find it as it did a great job of explaining the differences.
It has to do with the change from black & white at 30 fps to colour at 29.97 fps with non-drop being 108 frames too long over a one hour period.
Since 120 frames = 4 seconds, this would account for your “almost 4 seconds” difference.
I’ve posted this on the Sony Vegas forum to see if anyone agrees with me and I’ll let you know.
Mike
