Joseph morgan is signed on for season 4. Klaus was subdued por the Salvatore brothers and stuck in a coffin. However, when his life was threatened, Bonnie did a spell to protect her friends. So, when Alaric burned Klaus's body, his soul went into Tyler (sort of like how Klaus inhabited Alaric for awhile). Currently, he resides in Tyler which is why all the vampiros in Mystic Falls aren't dead. Because morgan is still on for season 4, we can assume he will somehow bet back his own body... which is good because the actor is amazing and the reason most of us fell in amor with Klaus in the first place.