Chapter I: The Ghosts Chapter: (Ep. 301 - 307) This chapter deals with the events surrounding Jeremy's new ability to see Ghosts and the mystery surrounding the return of Anna and Vicki to the world of the living. It also deals with Klaus's plan to create more hybrids as well as revealing his back story to his new wingman, Stefan.
The main antagonists of this chapter are Niklaus Mikaelson, Rebekah Mikaelson, and Stefan Salvatore.
Chapter II: The Originals Chapter: (Ep. 308 - 315) This chapter deals with the truth surrounding the creation of the vampires in the form of the Originals. It evolves into a conflict between Klaus and Stefan after the latter steals the coffins of the Originals and uses them in his crusade to destroy Klaus. It also sees the arrival and death of the one responsible for the creation of the vampires: Mikael, Klaus's stepfather. Also, vampires and humans alike are baffled when a killing spree begins against Founder's Council members.
The main antagonists of this chapter are Niklaus Mikaelson, Mikael and Stefan Salvatore.
Chapter III: The Bloodline Chapter: (Ep. 316 - 322) This chapter starts with the development of Esther's plot to destroy her family. Elijah and the rest of the family are not willing to just die and they use Elena to force Damon and Stefan to help them in their power struggle against their mother. Lastly, they learn that once you kill one Original, you also kill their bloodline of vampires, which leads to a search to find out who turned whom and which Originals they can kill without signing their own death sentences. It also continues the storyline of the mysterious killer who seems bent on killing the Council members who protect the secret of the vampires in Mystic Falls.
Chapter I: The Transition Chapter (Ep. 401-402):
This chapter also wraps up/coincides with The Bloodline Chapter from Season 3. This chapter focuses on Elena's transition into a vampire and all the things that she has to look forward to now that she is one. The newly reformed Town Council, whose mission is to clean the town of vampires and anyone else they deem a threat, decides to take action.
The main antagonists of this chapter are Pastor Young and Connor Jordan.
Chapter II: The Hunter Chapter (Ep. 403-409):
This chapter deals with a new threat that has arrived in the town of Mystic Falls; The supernatural hunter known as Connor has arrived to rid the town of all the vampires and hybrids bringing with him two main things, a new origin as one of the members of the thought to be extinct The Five, and also the key to revealing a Cure for vampirism.
The main antagonists of this chapter are Connor Jordan, Niklaus Mikaelson and Atticus Shane .
Chapter III: The Silas Chapter (Ep. 410-415):
This chapter deals with the origins of the first immortal Silas, and the Cure for vampirism. Jeremy tries to complete the Hunter's Mark to reveal the location of Silas and the Cure, and the journey to retrieve them. This results in tragedy and the raising of Silas.
The main antagonists of this chapter are Atticus Shane, Niklaus Mikaelson, Kol Mikaelson, Katherine Pierce, and Galen Vaughn.
Chapter IV: The Humanity Chapter (Ep. 416-423):
This chapter deals with the aftermath of the confrontation on the Island. After Jeremy's death, Elena's pain leads Damon to make a controversial decision which has dramatic consequences for all of Mystic Falls. Meanwhile, Silas, who is now awake, has followed the group back to Mystic Falls and is bent on accomplishing his ultimate goal: to drop the veil with The Other Side.