ScreenGems and Lakeshore are developing a fourth Underworld movie, so the feud between Lycans and Vampires will explode again. Underworld 4 will be shot in 3D and it's most likely a sequel, Producer Tom Rosenberg was already talking about it two years ago:
Rosenberg's quote is a bit dated, but more recently, Scriptwriter John Hlavin, who has been hired to write the script of Underworld 4, kind of confirmed that Underowrld 4 would be a sequel:
I hope Underworld 4 to be a direct sequel to Underworld 2: we'd see with Selene (Kate Beckinsale), who's more powerful than ever, and Michael Corvin (Scott Speedman), who's now a mighty hybrid, trying to clean up the chaos created by the death of the Elders.
Unfortunately this is still speculation, nothing official yet, but plot details of Underworld 4 should surface online quite soon! We will then have a better idea of what to expect in the movie Underworld 4.