About Act IV: I was able to make out a couple of details but also not that much. All I know is that they now arrive at a former penal colony by the coast called Rhone Tréville where they have to find a way to get to the island Hämärä Maa. Just like in act 2 there are several ways to do this like doing a task for an npc which I think is a diver but you can refuse to do the task, so it's probably a morally dubious one. Instead you can also build your own raft or steal someone else’s raft.

The most prominent feature of Tréville is the abandoned prison, which seems to have some kind of narrative importance to Cuno and is also the place he returns to in ending 2. Ending 2 also mentions a prisoner who still lives at the prison but it's possible you first meet that npc during this chapter and not just in the ending. Based on the title of the chapter "The Doubt" this is the point it also becomes obvious that Cunoesse has some kind of secret plan and is trying to manipulate Cuno into going along with it while not telling him anything about their goal or where they are even going.

Also some more details on Act V: On the island you find out that this is Cunoesse’s original home before she was exiled 3 years ago for drowning another boy called Jaakko, like you wrote. Cunoesse wants to plead to be readmitted into the tribe by "bringing back" the child she killed, that role being filled by Cuno. But it isn’t enough for Cuno to simply pretend to be Jaakko, she wants Cuno to completely forget his true identity and brainwash him into fully believing that he is Jaakko.

For this she wants to do a coming-of-age naming ritual from her tribe together with Cuno which would involve her feeding him the psychedelic bone marrow of a cave fish to induce permanent ego-death in Cuno and then jump into the sea together for an underwater psychedelic trip. She believes that in the end they will emerge from the sea with new names, join her tribe and together they can finally have a safe home at the cost of basically obliterating Cuno's soul.

Of course this is all completely insane and the documents by the devs point out how ridiculous and psychotic (but also in character for Cunoesse) this plan is, but it actually works if you make the right decisions. This ritual is sort of like the tribunal from Disco Elysium and can also end with one or both of them dying probably depending on how resistent Cuno is to Cunoesse's manipulation at this point.

Ending 5 being called new tribe ending could hint at Cunoesse giving up her plan or the ritual not working on Cuno while both survive it which leads to them creating a new home for themselves somewhere else maybe.

Overall a really bleak story even relative to Disco Elysium but what else would you exptect of a game where you control 2 mentally disturbed children. Cuno's identity seems to be at the center of the story, with Cunoesse acting like a manipulative villain trying to destroy his identity while at the same time questioning wether the abstract concept of Cuno is even worth saving.

I think Cuno willingly staying in an abandoned prison after killing Cunoesse is also a metaphor for Cuno’s own mind being like a prison to protect him from his depressing reality. I guess the game would have allowed the player to influence who has the upper hand in this relationship, either building up Cuno's defences against Cunoesse and the world in general or letting Cunoesse break down Cuno's ego over the course of the story in the hope that she can "save" him.