WARNING: series ends with a cliff hanger (-_-). I prefer to think of Torchwood, season 4 as "Torchwood USA". The season was co-produced by BBC One and the Starz network making it notably different from other Torchwood seasons, but not the full on American remake it was planned to be. Minor spoilers contained within the recap.
Season 3 of Torchwood was produced as a 5 part mini-series dealing with an attack on Earth and Torchwood.
Six months have passed since the attack of Abbadon and Jack running off with The Doctor.
Torchwood exists in the universe of Doctor Who. Even though the two share a universe, they hardly share the same parental guidance rating.