With the dramatic endings of season three as well as 'Children of Earth,' which aired on BBC America back in the spring in which we lost several Torchwood members, it's unclear exactly how Davies is going to revive the Torchwood organization. Captain Jack Harkness (Barrowman) himself had a dramatic departure, so it's pretty much a given that there will be a very dramatic return.
No official production date has been set for the return of the series, but creator Russell T Davies previously indicated he expected it to get started early in 2010.
Barrowman also told the programme that there were no current plans for Captain Jack Harkness to encounter the 11th Doctor, played by Matt Smith, who takes over the title role in Doctor Who from David Tennant in the New Year's Day special, The End of Time, Part 2.