There have been rumours for a while that Kate Beckinsale would return to the Underworld franchise for part 4, after missing part 3 (which admittedly wasn't really her fault, as it was a prequel to the other films, and her character was much younger in that one), and now that's been confirmed, with Variety reporting that it's been officially announced Beckinsale will be back as the slinky vampire Selene in Underworld 4.
There's not too much word on the plot, other than that it will continue the story from the second Underworld movie (and therefore long after Rise Of The Lycans). A director is currently being sought, with the producers also looking for a young actress to play Selene's daughter. It's being suggested that the film will see Selene passing the torch of trying to resist the war between Vampires and Lycans to her children.
The current plan is to shoot the movie next year for release in 2012