hmmm ... looks like the quest was indeed changed, if that's the mob it is telling you to kill on step 4. It used to be this one ...
4. Kuir-Afat, The Custodian. (These are the guys you kill for Shadowbreeds, in the Penumbra Dungeon, by Blue Mist) Yes again.
Note the "Yes again" at the end. That is added because this is the very same mob you killed in step 1:
1. Kuir-Afat, The Custodian. (These are the guys you kill for Shadowbreeds, in the Penumbra Dungeon, by Blue Mist)
So, I would guess that FC finally got around to fixing this quest, inserting the mob in the 4th spot they had originally intended. Now the question is, where is he?!!
I haven't looked in Inferno myself for the mob you mentioned, but there are 3 places he may be: Sorrow, the redeemed village, and the redeemed temple area both outside and inside. It would be unusual for FC to place a custodian in the temple itself, but you may want to check to make sure they didn't add one there.
The Pen quest had you go back to Adonis redeemed temple to kill a guy. Perhaps the Inferno sanc quest is the same, and points at a mob not located in Inferno.