The central mystery of Season 4 is identity of the Benefactor. Assassins receive cassette tapes with instructions on how to participate in a hit list called the deadpool, where they can earn rewards for killing supernatural creatures in Beacon Hills. 

The Benefactor arranged for Kate to open the Hale vault so that he could steal the Hale's fortune, $117m which he is using to fund the deadpool.

At the end of 4x09 - Perishable, it is heavily implied that Meredith is the Benefactor. Her suicide was faked, with the help of Brunski. And she tells Lydia, Stiles, and Parrish that Brunski wasn't on her list but he was a bad man.

In 4x10 - Monstrous, we learn that Peter Hale is the Benefactor, only he doesn't remember. While he was in a coma after the fire, he planned the entire thing and Meredith could overhear him. She enacted his plan.