When you put gandalf in play, and use the 4 damage response - do you factor that against the targets defense value also? Or does it skip the defense value?

no, it is 4 raw dmg.. so the defence dose not count on the mob you place the dmg on.... extremely powerful.

. .. using the word 'mob' in the way MMORPG-gamers do, meaning 1 enemy, not several.

Excellent thanks

That holds true for all damage effects that occur outside of combat, by the way. Hummerhorns deal 5 damage when they engage. That's just... what they do evidently. No defense, unless you can keep them in the staging area or kill them in the staging area.