[Rules] Assault hex recovery/retreat order

[rerathbun]

I think 3rd edition it was only one unit that suffered the free shot.  4th Edition it's all of them.

In 3rd edition the rule was the same.  Quoting from 3rd ed., rule 12.12 (see also 5.32):

If, during a given action segment, ALL of a player's combat units exit an assault hex (leaving no combat units to guard their retreat), then ALL the exiting units (not just the last unit to leave) may be assaulted by the enemy units in the assault hex using the Assault Table. This is a "free shot."