The film is canonically the sequel to the film Spiral (1998), which was ignored as a sequel and replaced with Ring 2 (1999).
Based on Koji Suzuki's 2012 novel "S". It is a loose adaptation of Suzuki's fourth novel. The novel is in the same continuity as the Spiral novel, which was adapted into Rasen (1998), but this film doesn't have any coherent continuity with Rasen (or any of the other Ringu films). That's partially because Suzuki's third novel, 1998's Loop, was never adapted into a film, leaving a huge gap of story in between these films.
The error message "404 File Not Found" is linked to the cursed video online. In Japan and some Asian countries, the numerical digit 4 is considered an unlucky number and is associated with death (due to the words 4 and death sounding similar in their native languages). The fear of the number 4 is known as tetraphobia and is taken seriously in some Asian countries that sometimes the number 4 is avoided in homes, hospitals, businesses and other properties.