Bloody Disgusting is reporting this as a rumor because the studio has not officially confirmed this. However, the two sources have spoken, and this gives them enough reason to believe that it will happen.
This is great news. When I heard that Kate Beckinsale was not going to be in the third movie, it took all the wind out of the Underworld sail. I didn't even go see the movie. When I rented it however I was really impressed.
But now news that Kate will hopefully be back for the fourth Underworld movie has got me all happy again. It is also said that this movie will be released in 3D. With today's technology it will be a smash you in your mouth hit.