Entertainment Weekly reports that the new episodes - which will be co-produced by the BBC and US network Starz - will introduce new regular Rex Matheson, a "CIA agent born to make waves" with a "wicked" sense of humour.
The show will also introduce two new recurring characters - Oswald Jones, a convicted murderer and paedophile, and Esther Katusi, a young CIA agent "who learns what she’s really made of only when she’s forced to".
Both roles are said to be "key to the new season".
Casting for all three parts is believed to be ongoing.
Series star John Barrowman recently revealed that he is "ecstatic" about the fourth series of Torchwood, while his co-star Eve Myles admitted that she is "nervous" about filming in America.
The new episodes are expected to air on Starz in the US and on BBC One in the UK next summer.