Julie Plec has hinted that another main character will die permanently at the end of Season 4. Here's what I think:
Can't be Elena, Stefan or Damon because they are the 3 main heroes of the show.
Can't be Klaus, Elijah or Rebekah because there's a good chance that they're leaving for New Orleans at the end of the season (The Originals spin-off).
Can't be Caroline because Julie Plec said that Season 4 is not the end of Klaroline, and I don't think they would kill Caroline considering that Klaroline is the most shipped couple on the show.
So there are 4 main characters left: Tyler, Matt, Bonnie and Katherine.
I don't think Tyler would die since it's been said that he's not coming back to Mystic Falls for the rest of the season, so I'm not sure how his death would be relevant.
Matt could die, as he is a human and even if the other side gets dropped he won't come back. But I'm not sure what purpose his death would serve but it could be him.
Then there's Bonnie and Katherine left. I'm pretty sure it's one of those two that will die. Because as it's been said Bonnie will force Katherine to help her defeat Silas and then when they try to defeat him something will go wrong somehow and one of them dies permanently after the other side gets destroyed.
I could be wrong but I'm pretty sure that Katherine is the main character who will die. Time will tell. But I'm pretty sure it's not gonna be Stefan/Elena/Damon or Klaus/Rebekah/Elijah or Caroline.