The dungeon level whose changeling population Juice Box's brother was questing to free.
Carl remembers that Juice Box's Brother was on a quest not only to fill The Changeling Library by touching rare creatures, but first and foremost to get his people off the fourth floor. That rescue effort was the changelings' core objective before his death on The Fifth Floor.
The detail contextualizes Juice Box herself: her family's involvement in the Crawl began as an attempt to extract their people from a level of the dungeon, not as a bid for fame or power.
The dungeon level holding the changeling people Juice Box's brother tried to liberate.
Also appears in books you've already read — spoiler-safe to their endings: