Torchwood Four was a branch of the Torchwood Institute. The nature of this branch and its personnel was unknown. Bilis Manger's handprint gave him top access at Torchwood Four. (PROSE: The Twilight Streets)
Torchwood Four disappeared (TV: Everything Changes) thanks to Object 1. (AUDIO: The Torchwood Archive) Agnes Havisham claimed that Torchwood Four's disappearance was a "messy business". (PROSE: Risk Assessment)
Suzie Costello had reports documenting her search for Torchwood Four. (PROSE: The Twilight Streets) She investigated a black spot off the coast of Iceland in hopes of it being the missing branch, but found a crashed spaceship from the 32nd century there instead. (AUDIO: The Last Love Song of Suzie Costello)
Jack Harkness later declared "we'll find it some day", when he explained Torchwood to Gwen Cooper. (TV: Everything Changes)
Jeremiah Henderson was briefly shown part of the fall of Torchwood Four whilst visiting the Torchwood Archive. (AUDIO: The Torchwood Archive)