This is just speculation but in the latest episode the "Deal", Bonnie kept having recurring dreams of a coffin and the fourth one when she opened it in her dream, she saw Klaus and he was holding the Original Witch's necklace. I am thinking that it is the Original Petrova in the sealed coffin that Stefan cannot open. He told Bonnie that he tried everything to open it with i.e., blow-torch, crowbar, etc. and it will not open. Bonnie stated that it is most likely sealed with a witch's spell. I remember in episode in Season 2 when Elijah met Katherine for the first time and while staring at her he said that she reminded him of someone. Also, I read somewhere that Katherine is not the original Petrova but there was another one before her by the name of "Charlotte" Petrova, or something like that. What do you think? Is there a body in that sealed, fourth coffin? Or something else that will help Bonnie in finding a way to help Stefan defeat Klaus?