Isao Yatsu, who plays Saitô (the old man in the wheelchair playing "peekaboo" with Toshio) in the third movie (he would later appear in The Grudge 2 playing almost the same role), previously appeared as the apartment manager Kamiya in Dark Water.
Kobayashi is played by Yûrei Yanagi, who appeared as Okazaki in the first two Ring films.
Kamio, one of the detectives who appears in the first two movies, is played by Tarô Suwa, who appeared in the extended version of Battle Royale as the middle-aged paedophile.
Name's the Same: Confusingly, the original titles of the third and fourth movies are simply Ju-on and Ju-on 2... which are the exact same titles as the first two. The sub-titles The Curse (for the first two) and The Grudge (for the third and fourth) are usually added in foreign markets to differentiate between them.
The Other Darrin: Toshio has been played by four actors in total throughout the entire Ju-on saga, including the short film 4444444444. In the aforementioned short, he was portrayed by Daiki Sawada. In the two V-Cinema films, he was portrayed by Ryōta Koyama, and in the two theatrical films, he was portrayed by Yuya Ozeki (who would then go on to reprise the role for the first installment of the remake series, as well as in flashbacks for the sequels). In Black Ghost and White Ghost, he is portrayed by Shūsei Uto.
In Ju-on: Beginning of the End, Misaki Saisho has replaced Takako Fuji as Kayako, and Yasuhito Hida has replaced Takashi Matsuyama as Takeo.