Klaus is not dead. Julie Plec has made this clear in multiple interviews since The Vampire Diaries Season 3 finale aired.
But she's also teased a new villain on the way for season four (which will premiere in October, The CW announced today, along with its upcoming schedule) and a new network press release makes the same mention.
It reads, regarding what's ahead:
Season Four begins with the knowledge that everything is changing. As Elena and her friends enter into the final stretch of high school before graduation sends them off on different paths, their bond to their home town of Mystic Falls takes on a deeper meaning when a new villain seems intent on destroying it.
React now and be sure to get your first look at Beauty and the Beast, which will follow TVD this fall on Thursday nights!