“I can tell you it’s not a prequel. It will satisfy old fans and excite new audiences, meaning that we don’t want to redo the first three movies, so steps are being taken to honor what fans have loved but at the same time introduce fresh elements.”
Scriptwriter John Hlavin
So if it’s neither a prequel nor a reboot, then we may expect a normal sequel that pick up where the story left in Underworld 2. Anyway, more plot details of Underworld will probably leak online sooner or later.