The mysterious resident of room #4.
The other tenants gave up on finding out anything about this strange man whose personal information is completely unknown, even job or age. Yet he seems to have something to call his job, and somewhere to call home, since he was away during the new year's holiday.
His hobby is peeping and eavesdropping. His room's closet is connected to Godai's room by a huge crack that Yotsuya always opened whenever somebody tried to seal it. The only time anybody saw Yotsuya at work was when he tutored Yagami, although he didn't teach her anything. All he has done in the course of four years recorded in TV or manga has been to create unspeakable messes.
It has been speculated among fans that Yotsuya is one and the same as the character Sekoi in another Rumiko Takahashi Manga titled Dust-spurt. In it, Sekoi is a secret agent working for the HCIA="Hinomaru Central Intelligence Agency."