So, it looks like we have an agreed errata for page 2 of the adventure book, that adventure 1-4 is followed by Yawning Portal, 5-8 by Skullport, and 9+ by the Depth. This is also in line with the instruction stated in aftermath of Adventure 4, which says that you can take Yawning Portal actions after this adventure.
But... the fluff text describes that, using Durnan's map, we have just arrived to Skullport, with it's shady and dilapidated buildings.... and then we take Yawning Portal actions? A little confusing thematically. OK, maybe the Yawning Portal actions are taken BEFORE this, when we return to Durnan with news about the Halaster rumours, and get his map. But then it would have been better to put the thematic lines about arriving to Skullport at the start of the next adventure, as opposed to having it here.
Anyway, I still believe the errata is correct (1-4: Yawning Portal, 5-8: Skullport), just wanted to mention that the description is confusing.
I just finished adventure 4 and I agree with you. I think the flavour text about arriving in Skullport should have been put in at the start of the next adventure since they flagged Skullport as adventures 5-8.