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It's hard to say exactly without seeing the chapter. Maybe there was a major plot twist or a key character's fate was revealed.

In Chapter 4, the main character faces a major setback that threatens to derail their mission. There's a lot of action and suspense.

Todoroki might be another likely character. He has a complex power set and has been through a lot of emotional and physical growth. His struggle to control his two quirks could be a parallel to the control needed for Ultra Instinct. If he can master the balance within himself, it could potentially lead to the awakening of Ultra Instinct in him.

The Ultra Instinct in a fanfiction might also have an impact on the character's growth. It could be a new milestone in their hero journey. Instead of just relying on their existing quirks, they have this new ability to master. This could lead to new relationships and rivalries. Other characters might be in awe or jealous of this new power, which would create more drama and tension within the story.

Maybe it's a feature that was automatically added or a glitch. Hard to say for sure without more context.

Midoriya and Toga. It's an unexpected pairing because Midoriya is the symbol of hope among heroes and Toga is a rather blood - thirsty villain. But if you think about it, Toga's admiration for heroes could be a starting point. Maybe she could be redeemed or at least have a moment of self - reflection through her interactions with Midoriya.

Maybe you encountered a creepy shadow in an abandoned building.

I think Mineta and Momo would be a really unexpected pair. Just imagine Mineta in Momo's body having to act in a more ladylike and intelligent way, which would be a huge contrast to his normal perverted self. And Momo in Mineta's body would have to deal with his short stature and his rather... unique personality traits. It would be quite the comical and challenging situation.

The Powerpuff Girls. Blossom, Bubbles and Buttercup in the world of My Hero Academia would be amazing. They could team up with the female heroes like Uraraka and Tsuyu. Their superpowers could combine with the quirks in some really unique battles against powerful villains.

In a fanfic scenario where Naruto consumes the Rumble Rumble fruit, it would be a huge game - changer for him. His chakra nature could potentially be affected, and he might be able to generate electricity from his own chakra. This new power could help him in battles against stronger opponents. He could send out electric waves to disrupt enemy attacks or even paralyze them. Moreover, it could also change his relationships with other characters as they would have to adapt to his new - found abilities.