There’s a lot less monster variety and monster ecology in action in book 4. The reason to this is that Laios’s guild is now at the 5th Level (the City) and will encounter its deadly new resident, the Red Dragon, who (we can safely guess) pretty much chased off everything else (except ghosts). Of course its also the kind of encounter that can take quite a few pages…

(As usual, I’m keeping the spoilers to a minimum)

• The book start with a scene with the richly-attired governor of the island (from the village above the dungeon) and a couple of gnome magic-users that serves as his advisors. One thing to note is that the plundering of the dungeon has brought tremendous wealth to the village, probably more within the first few years of its discovery. Now that it doesn’t seem so treasure-filled what’s still of great value (to some people at least) is its secrets. The Biggie being the special magic that binds soul and body. The governor is thus encouraged to continue to pay adventurers (as loot isn’t so prevalent anymore) to get below and kill monsters so that others can do their research with less risk.
• The City Level is still mostly in good state. Its 3-4 story high buildings are densely packed, in proper medieval-style, with many covered bridges crossing above the streets.
• We already knew that the Red Dragon has displaced an Orc tribe, we now see dead wargs, incinerated by fire breath.
• Dragons have only one weak spot, on the neck, just below the jaw. Their scaled skin is otherwise impervious to normal weapons (non-normal include those made in adamantium and mithril).
• We see a demonstration of forbidden Ancient Magic. The Elf Marcille, its user, argues that it’s what we make of it that counts. Chilchuck the Halfling counters that it hasn’t been forbidden for nothing, he also says (not to her face) that he hopes that she isn’t a « dark elf », obviously not a racial thing in this context.
• We see someone asking hospitality from a gentle-looking ghost of a young girl that agrees.
• We learn that wargs are ferocious, that a pack can hunt much larger prey, that they’re very smart but also that they live with orcs of their own volition and not because they are trained by them.

Book 5 Here