4x01 - The Dark Moon
Derek is sealed behind a wall in a la iglesia inside a nagual temple to Tezcatlipoca by Kate. When Scott and Braeden rescue him, he's been de-aged. He is now 15 years old.
4x02 - 117
A younger Derek reutrns to Beacon Hills. He goes to the Hale house. He doesn't remember the fire that burned down the house and killed his family. It is revealed by Peter that Kate has sent Derek back to the age where he trusts her. Kate convinces Derek to open the Hale Family Vault underneath Beacon Hills High School. At the end of the episode, Derek returns to his proper age. For unknown reasons, his eye color has changed back to yellow.
4x03 - Muted
Derek hires Braeden to find Kate. He hopes that he can learn what Kate did to make his eyes changed from blue back to yellow.
4x04 - The Benefactor
Derek helps Sheriff Stilinski find The Mute. They search the school. Sheriff Stilinski finds a pool of blood. Derek says that's impossible, claiming he would have been able to smell it. The Mute is at the school. They fight him until Peter shows up at kill him.
4x05 - IED
Derek meets Liam and advises Scott on how to teach Liam control. Derek later shows Chris the Hale family Vault. He explains that Kate did something to change the color of his eyes and to lose his powers.
4x06 - Orphaned
Derek and Malia go in search of Satomi's pack. They find all of the pack dead. Among the bodies, was an alive but severely injured Braeden.
4x07 - Weaponized
Derek takes an injured Braeden to the hospital and stays to watch over her.
When Melissa wakes Braeden up to get an explanation of what happened, Derek interjects that the werewolves were poisoned, but Braeden corrects that they were infected by a virus designed to kill them.
Later, Satomi joins them and they go to see Deaton in the morgue for an explanation. Deaton explains that the illness is a variant of canine distemper that has been modified to act faster. Derek adds, "You mean it's been weaponized" to remove Deaton's vagueness. Satomi was not infected, meaning she either wasn't exposed to the virus or is immune.
Derek asks Satomi who was shooting at the entrance to the woods. "Apparently another assassin. Personally, I'd rather face a gun than a variant of smallpox." Melissa replies, "Sounds like you're going to get plenty of chances."
Satomi turns to look at Derek, and he catches her looking. "Sorry. I just noticed how much you remind me of Talia." Derek looks away, a few emotions flitting across his face. She is the second person, after Deucalion, to remark about Derek reminding them of Talia and foreshadows that he will earn the full wolf form and has the potential to be a well respected alpha like his mother.
Satomi says she used to visit Talia a lot. She asks if he remembers her. "I remember the tea. You always used to bring that tea that smelled terrible." Satomi says that the tea was a gift because Talia loved it. Deaton asks what kind of tea. "Reishi. Wild purple reishi. It's very rare." "It's also a very powerful remedy for sickness." Satomi didn't get infected because she'd been inoculated by the tea. Melissa perks up and asks how rare the mushroom is. "Can we find it?" "We don't have to. My mother kept some of it. It's in our vault."
4x08 - Time of Death
4x09 - Perishable
4x10 - Monstrous
4x11 - A Promise to the Dead
4x12 - Smoke and Mirrors