I saw a bar fight (fists only) and one of the actors did a boxing "slip" (a type of duck were his head weaved under the attack and then came back up with his own punch).

Was that just an (annoyingly unfair for players) scripted animation or is that a move that the player can some how use?

Sure we can BLOCK (silly crossed arm thing but that seems to be the only kind of "block" all game developers know) in a fist fight, but I have found no way to DUCK in Fallout 4. I thought I would have to mod this in myself.

However if this is already in the game for the player I will be very pleased with Bethesda for this (and that will will raise my personal rating of this game).

As far as I know, it is an NPC only animation. Though the player does get special animations sometimes if you kill an NPC or enemy with a power unarmed attack.