Quick Summary

Hina "wins", with Hoshino spending most of the fight trying to run away. Hina reveals she investigated Yume's death as a first year and it really was just an accident, it's not Hoshino's fault. But the Dweller is constantly whispering in Hoshino's ear and she still thinks she killed Yume, and that she wants to die herself. She becomes Hoshino Terror aka Horus, and attacks Hina leading to an image similar to the Vol F CG of Hina on the ground (no blood). Sensei and the taskforce defeat Suou off screen but the entire world is ending thanks to Hoshino Terror, so they go to stop her. Dweller reveals this was the entire plan from the start, he wanted to break Hoshino, so he had won the game. Set shows up as Horus's natural enemy and the Dweller celebrates the end of the world as they are about to fight

Plana says the way to stop all of this is to break Hoshino's halo, but Sensei flat out rejects the option. She says another force of Terror could face her but there's nothing like that there. Hoshino Terror attacks Sensei, Plana and Arona barely manage to save him but he gets knocked out. Shiroko hears the Chroma calling to her and begins to reach out to it for the power to stop Hoshino

Sensei in an unconscious state somehow drifts to another world (you actually see/hear the prologue train), waking up where he was already in Kivotos for Yume and first year Hoshino. He apologizes that he can't save Yume, he can't rewind time or bring people back from the dead. All he can do is stretch out a hand to students in need. She's confused but tells him sometimes that's enough, to keep believing and that miracles can happen. He wakes up in the Dweller's world and introduces himself, telling him the "game" isn't over yet. He then calls out for Shiroko to stop Shiroko, and Shiroko Terror teleports in to stop Shiroko from reaching out to the Chroma. Then she goes to face Hoshino Terror and the part 4 ends.

Show me a good time, Takanashi Hoshino (1/4)

Hoshino never calls Hina Hina, only referring to her by her job title + -chan.

Hoshino snaps at Hina like "Why do you think you're involved? Because of Sensei?" and Hina is like >:( but later also reveals that she has vowed, basically, to get rid of all the Thunder Emperor's old weapons (along with Makoto). She says she didn't know about the weapon, and Yume must not have either. Her death was an accident and not Hoshino's fault.

Hina says she understand Hoshino after Hoshino gets mad at her, saying "You think you know me?" Hina says she understands Hoshino very well due to learning so much about her, and Hina yells at her when she clams up--shouting "Don't TELL me you don't understand!" after she explains that Sensei can solve their Kaiser problems, somehow. She says she'll keep Hoshino in place until things settle down.

Underground Man (not Dweller. It's not "Basement Dweller") begins reminding Hoshino of the notebook. The objective of this is to drive the point home to Hoshino that Yume is dead. The lack of the notebook = a lack of closure. With coaxing, The Underground Man gets Hoshino to blame herself for Yume's death, in the greater sense that even reaching her hand out to Yume in the first place was a mistake. She wants to kill herself, and feels she is Yume's killer. That's when she goes Terror.

Show me a good time, Takanashi Hoshino (2/4)

Suou is meanwhile offscreen'd by the other Abydos girls, stopping her from using the Shemata. The Shupogaki helped them get there. Sensei and the rest witness Hoshino exploding with power. When Sensei gets close, his immediate question is "Hina! Are you okay!?" and he runs to her. While looking at her body, he gets a flash"back" to Hina's death--which he first received in volume F.

Shiroko specifically says she understands what she has to do and that she was basically born for this moment. She calls to the Color, it doesn't call to her as in the summary. This is what happened with Kuroko too. She reaches for it.

Show me a good time, Takanashi Hoshino (3/4)

After Hoshino Horus takes Sensei out, Sensei starts feeling like absolute shit--similar to volume 2 chapter 2. He's said this before many times, but here his acknowledgement that he's literally "just some guy" is ruining him. He can't stop Hoshino and he can't bring Yume back nor turn back time. We see a series of flashbacks, going all the way back to Sensei's first day in Kivotos, and then indeed the train. After that, it's not really a flashback, but you're speaking with Yume and Hoshino is...somewhere else, but yelling into the room. Yume gives no indication that she's dead or the situation is strange, but Sensei is horribly depressed. Yume cheers him up, and after Sensei says "I just reach out when I can" Yume says that's perfectly enough, and that little acts can eventually create a miracle (and then goes cute because she knows talking like that annoys Hoshino). Sensei vows to remember her words, and Yume says she hopes she's able to help.

When he returns etc Shiroko Terror specifically says that Shiroko shouldn't touch that thing. She also "Nn"s (ん) for the first time since becoming "Kuroko". She explains and greatly hints that she has killed Hoshino before, as she says she's beaten Hoshino before and destroyed her halo. She also tells Shiroko that "you're me, so I know you're strong". I personally don't imagine she'll aim to kill this time, but she does say she won't lose.

Show me a good time, Takanashi Hoshino (4/4)

Hina says "Surely your friends at Abydos and Sensei told you all this already, but you probably feel like you have to fight things all by yourself." which prompts the "You think you know me?" from Hoshino. She says "Yeah I do, and I know how you think because etc etc." she also adds "Yes. You're stronger than me." because she believes that Hoshino staying in Abydos and holding her ground was very impressive, and something Hina believes she couldn't have done

We do get confirmation that Hoshino "named" Black Suit, as he never bothered choosing a name in Kivotos