Chapter 4 is a chapter that diverts from the game's platforming elements to deliver a unique gameplay theme, and is also the first Chapter in which analysis of the story as a "Town Story" makes sense.
The major plot of the chapter is centered around the mystery of Squirps, an alien who answers precisely zero of the questions asked of him, and the information he does choose to share is vague, delivered in an impenetrable vocal pattern and often can be learned with a cursory look at the surroundings. Squirps is meant to be frustrating, but the game runs head-first into an issue of tone holisticially.
Thus far, the game has typically been a light-hearted romp with humorous NPCs to talk to, charming villains to bumble about in our path, and has kept the intrigue localised entirely to parts of the narrative it keeps openly mysterious, like the love story between Blumiere and Timpani and Dimentio's scheming. Chapters 2 and 3, which had serious plots but no actual serious moments to underscore them, set the tone of what a "serious plot" looks like in SPM. For this chapter, Tippi and the Heroes constantly raise suspicions of Squirps, setting up this vague and unhelpful character to be antagonistic, but the player is not convinced in the slightest. Not because they suspect Squirps is deliberately unhelpful- they're not convinced Squirps is evil because a helpful character who speaks the way Squirps does fits right in with the tone of SPM. The characters raising red flags, written in such a way that it is more likely signposting than humour, is taken as more humour. When Squirps is revealed as a prince, son of the guardian of the Pure Heart and last hope for her will that it reach the Heroes, the audience is not surprised because "I thought you were evil", like Tippi- they honestly just kinda roll with it because that's what Squirps is doing.
As a defense mechanism for the Pure Hearts, Squirpina's method is rather interesting. She locks the Heart in the bowels of the least pleasant place in the cosmos to go to, seals her son in hypersleep to serve as a guide once he arrives, and then... lets the treasure be forgotten. It's not in danger because no one knows it's there, nor are they interested in going there for the sake of Treasure. As a concept, the plan was rather sound- Bleck's minions didn't get much of an ambush in here, more of a follow- but Squirps did not come out of hypersleep with a helpful mental state- perhaps she thought Squirps would mature while in hypersleep, but forgot maturing requires actually doing things.
Speaking of Squirps, I find it interesting that dealing with Squirps involves two things that RPGs typically shy away from: pooping, and eating. It's a little weird that it is delivered in that order (you'd think feeding him chocolate would cause him to need the potty later), but it says a lot about RPGs, as well as society as a whole, that "the plot involves helping a character use the bathroom" is inherently a comedic skit. I will give the game some credit for not making the joke about Squirps not wiping itself.
On a completely unrelated note, the chapter's villain and Team Bleck encounter. Mr. L is Super Paper Mario's biggest impact on the Mario IP, only not being the game's most memorable aspect because Count Bleck and Dimentio are poised to do that themselves later. Starting with the N64 era (possibly due to his omission from Super Mario 64), Luigi has been the butt of every joke, the eternal second banana, and Paper Mario is no exception. While Luigi is not a real person and bullying him does not have consequences on his psyche, the many jokes at his expense cultivated an environment where "Luigi has enough and bites back" was an obvious joke that somebody was going to snipe at, and many fans did. IS, on the other hand, took it upon themselves to pull the trigger and put it in an official game, in the sort of game openly willing to take these risks, and alongside the existence of the Negative Zone in Super Smash Bros. Brawl, Luigi's dark side rose to canon status (as much as Mario has such). With it out in the open like this, the path that led to Dream Team's reclamation of Luigi as the proud eternal sidekick was laid out.
With that said, in Chapter 4 itself, Mr. L is incredibly underutilised. He shows up only at the very end, having absolutely no connection to the Squirps story. He has followed us into the depths of the Space Graveyard, despite the narrative promising this is a difficult feat, and challenged us to a boss fight with nothing but a quick establishment of who, exactly, the Mysterious Mr. L is, as a character in his own right. His Brobot boss fight, being in space with the Squirps gun, feels like something that should've existed earlier in the chapter, setting up Mr. L's cred as a villain alongside the other members of Team Bleck. Clearly they wanted the surprise reveal, but they also ruined that by calling 4-4 "The Mysterious Mr. L" and revealing his presence, if not his identity- and even then, when Luigi has been as blatantly removed from the plot as he was, a character identifying as L is likely Luigi anyway. Mr. L's largest contributions to the narrative are his commentaries on Luigi's characterisation, not his actual... contributions.
Mechanically, Chapter 4 is of course a curveball chapter. With 4-1 and 4-3 being entirely shmup exploration and 4-2 using low gravity to make comparisons to the rest of the game difficult, this is a breath of fresh air (ironically) that the gameplay loop did kinda need. Since Chapter 6 uses a similar non-standard setup, I suspect there is an element of intentionality to this, but it's much easier to fall into just focusing on the story aspects when we're able to be distracted from the gameplay mechanically as well as narratively, as incredibly hilarious as that sounds. While the chapter is filled with the same mechanical issues as the rest of the game, the exotic theming helps make them more bearable and wacky. Leaning into the issues rather than pretending they don't exist, as it were.
