Torchwood Four

Torchwood 4 was a branch of the Torchwood Institute. The nature of this branch and its personnel was unknown. Jack Harkness stated it was "missing," and declared "we'll find it some day." (TV: Everything Changes). Agnes Havisham claimed that Torchwood Four's disappearance was a "messy business". (PROSE: Risk Assessment)

Given the locations of Torchwood Three, Torchwood One and Torchwood Two dispersed across Great Britain it is plausible that Torchwood Four was located in Northern Ireland