I think Chapter 4 is one of the best chapters of the campaign.
City of Gracklstugh, by far the most civilized place PCs can visit during their runaway, is well described, with many informations about landmarks, history and so on.
The map is gorgeous and there are many NPCs and many factions as well.
You could meet Duergar, Derro, Stone Giants, Drow and even a Red Dragon who fuels the city's furnaces.
Many well detailed encounter tables and side-quests complete the city's description.
NPCs and factions may be too many to handle with and you may get lost among all the plots. If you want less and easier work, focus about a faction a time; PCs will likely return there again and again, so you could follow the other plots during the following visits.
You'll meet new rules to apply while in Gracklstugh and many Duergar showing signs of madness.
Focus about Duergar's madness; PCs must realize things are getting very strange.
While in the city there are high chances PCs will be jailed. Don't worry: it will be only the beginning of a lot of events, which, according to how good will be played by them, will eventually end with their leaving from the city, in hiding or with full honours and respect from the Duergar.
As a reward for quests accomplished you could allow Duergar to give PCs a "complete" equipment. At this point of the campaign some of them might still need it.
Outside Gracklstugh, new other bizzarre places are waiting for the PCs.