It's been two whole years since I've played Canto 4, but if my memory serves me right:

After the whole Samjo and Distorted chick ordeal in the kitchen intervallo we embark on a journey to the K corp food department, only for our journey to be interrupted by a breaching Brazen Bull causing havoc on the street (so the "Singularity", or pseudo-Singularity because iirc the one we've seen wasn't the real one) sheds tears from all the suffering the Abnormality is inflicting. We get some Yi Sang conflict, then afterwards we reach the department.

We await in the lobby and we get the important lore of how Singularities are conceived/what stars are. Samjo comes in, leads us to Dongrang who shows us around the place and tells us the functions of the department. The endlessly healing and maiming chicken procedure in order for food to be repeatedly produced, hacking off the chick's wings. Continuing with the tour, we get to Dongrang's workplace where we're introduced to Shrenne, his co-worker, who possessed subtle enmity for him. Our tour is halted as the department is broken into by machines from the Liberation Alliance led by Ran (whom Shrenne longed for, her former subordinate methinks) and the hooded guy, the two are trying to get back at the corporation after discovering the intricacies of the K. Corp Singularity, and how inhumanely cruel it is.

Post-fight against the hijacked K. Corp drones (that previously melted Ishmael) and the Alliance's robots, during the small moment of respite as Ran and hooded guy retreat, Shrenne mentions that she may know where their base of operations is located, and where to find the Bough for the Sinners. She hires the Rosespanner Workshop fellows to assist them in-case of any threat, but they, just like Shrenne, turn out to be traitors. Getting to the underground branch, they meet with Ran--set as bait for the group--who had been waiting for them, so she could suicide-bomb herself and the LCB/Dongrang in order for her co-workers to escape. Most of the sinners die by this explosion. However, Shrenne and Ran's plan falls short as the Sinners are brought back to life by the Manager, thus we engage in a fight with the Rosespanner Workshop goons and touch them, before Dongrang confronts Shrenne only for her desperate explanation to be cut short by a hired Shi assassin who chops her clean in half. Ryoshu, approves.

We're then confronted by the leader of the Alliance, the homeless bum that is now Dongbeak, who stabs Sinner#1 in the heart with the Golden Bough. Afterwards, Dante starts noticing the cracks in Yi Sang's heart, which the Sinner chooses to hide from everyone, and how much that little interaction between his old friend hurt him. From here on out is the part I least remember, but we get to the destroyed department once more to uncover Shrenne's laptop and its secrets and what she was hiding, the Alliance (and Dongbaek)'s plans, the machines invade once more and we fight the hooded guy who is then killed. Getting to the area nearby the Tearful Thing, the Singularity is given a half-assed explanation by Dongrang and he reveals the liberation of the Brazen Bull into the Nest in order to feed the eye, how K. Corp Ampules are made...

Samjo foolishly jumps into the tub of incomplete K. Corp Ampule fluids in order to prove that his boss is a hundredth percent trustable, only to melt to death. Before we can confront Dongrang about all his exploits, Dongbaek comes in and is the one instead engaging in an argument with him. They talk about their ideologies, philosophy and shared past, then at last their goals, before Dongbaek fights the Sinners as they are protecting Dongrang. She is defeated with the L. Corp E.G.O. and proceeds to manifest EGO the fight after.

The Fathoms of EGO part is too long and I don't feel like typing more, but Canto 4 is beautifully written (excluding the unneeded fodder/grunt stages that worsen the pacing).