Part 4 covers less than a month (most of the 9th month), but contains fifteen entries.

On the 1st day of the 9th month, the narrator plucks scraps of paper from the gulls’ nests and takes them back to his own hall to piece them together. He puzzles together a page of writing about being imprisoned in the House. However, the “slavery” described doesn’t seem to fit in with his recalled experiences of autonomous fishing and cataloging, so he tries to figure out who wrote it. Piranesi later learns of his own imprisonment, but never fully regains those repressed or lost memories. The second page he pieces together contains writing about going insane, and both pages talk about a fight between people (which turn out to be him and Ketterley).

The following day, Piranesi wants to ask Ketterley about the named dead but doesn’t want to reveal information about his journals. Ketterley obsesses about 16 instead of working on his ritual.

On the 5th, the narrator smells something like perfume and tracks the scent for a while, but eventually loses it. From this smell, he realizes a new person—16—has passed through the halls, and tries to remain on guard despite the sweet scent.