I believe this content was added dec 6th so this post would have been before the content was added however still issues with this quest. To start sorry this is probably going to be long. I started my hunter (my main most content is completed on this one) changed to my titan got to the born in darkness part 4 and have not completed it on my titan. Back to my hunter and get to the part where born in darkness part 4 should be available. It is not. I can only assume as i did a bunch of stuff prior to doing my titan and i completed the lament on my titan something i did on my titan is not done on my hunter. I can only assume there is a prerequisite that is not shared across account but only per character. I have completed all i could find for destiny 2 as my hunter is my main its something i did on my titan i haven't yet done on my hunter. It would be new content. I have already picked up the salvations grip however i didn't receive a second gl after completion. I remembered on my titan i had 3 master lost sectors pop up independently of each other (i believe there were 2 and one left and a different one came) but didn't complete them just before the quest popped up those 3 indicators on the map were no longer there. My hunter now has one left perdition. I believe i farmed lost sectors to do most of the quest completions on my titan but farmed mostly gambit for my hunter. Im going to go to do the second quest next to Variks then i will farm the lost sectors until the master mark is no longer on the map ( the two are reforging the past (doing this one) and stealing stasis didnt open (part 4) key of truth). Other than that im trying to find anything thats not in my titans quests that is in my hunters.

I am in the same boat have you figured this out