Torchwood Four

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. At the time of Torchwood's foundation in 1879, Ireland was still a part of the United Kingdom, and as such, its capital, Dublin, would have been the most likely site of Torchwood 4's headquarters. The War of Independence (1919-1921) followed by the separation of southern Ireland from the UK might have had some role in the 'loss' of Torchwood 4.

Since it has been recently confirmed a fourth season of Torchwood has been picked up it could be entirely possibly that some mention or even Torchwood four it self may appear in the new series.

It's disappearance could be a reference to the disappearance of Babylon 4 from the US series Babylon 5.