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Torchwood 4 is, or was, a branch of the Torchwood Institute. The nature of this branch, its location, and its personnel are at present unknown. According to information passed from Torchwood 3 leader Jack Harkness to new recruit Gwen Cooper, Torchwood 4 is lost, though there is hope it may be found one day. (TW: Everything Changes)
Behind the scenes
No further information regarding Torchwood 4 has, to date, been provided on screen. Speculation as to what and where Torchwood 4 was includes the possibility that it was the base at one time located under the Thames (DW: The Runaway Bride), or possibly the parallel Earth Torchwood on Pete's World (DW: 'Doomsday).
Since there were/are Torchwood bases in each of the UK's countries; England (1), Scotland (2), Wales (3); perhaps Torchwood 4 is based in Northern Ireland.