kotori is that blond guy, hiyori. i suspected it from the very beginning but honestly i don’t know how he was sending these tweets in real time while being next to uta-chan. but yeah, basically hiyori must’ve been obsessed with uta-chan for a long time and because he didn’t want to tell uta-chan that, he just made a bunch of twitter accounts to simultaneously threaten uta but also “comfort” him. it’s a very common tactic of a stalker to make their victim feel threatened and in constant state of fear because it makes them feel in control. and because hiyori is, well, a little bit fucked up, he probably gets off to seeing uta in distress, especially considering the fact that the distress in question is hiyori’s own doing. at the end, to make himself look like a good guy and even a hero of sorts, he makes uta believe the teacher is kotori and “rescues” him from teacher’s stalking (again, the stalker is not the teacher but hiyori himself) by telling that he and uta are now a thing, so the teacher doesn’t have a chance. by saying that last bits about birds in different colors, he basically admits that he’s not only kotori, but also the other accounts that so far have been comforting uta and asking him if everything is alright. quite fucked up if you asked me, but not nearly as much as some stories out there.
can somebody please give me a summary of ch 4 because i didn't understand it at all