The fourth season follows the battles between Ragnar and Rollo in Francia, Bjorn's raid into the Mediterranean, and the Great Heathen Army's invasion of England.
Ragnar, wounded and near death, has a dream of walking towards the open gates of Valhalla, only to see them close before him. Aslaug asks the Seer if a woman would rule Kattegat, should Ragnar die. Bjorn has Floki arrested for killing Athelstan. When Ragnar awakens, he chastises Bjorn for imprisoning his friend. Bjorn wants to prove himself by surviving deep in the wilderness.
Aslaug procures a new slave girl, Yidu. In Hedeby, Earl Kalf announces that he and Lagertha will share the Earldom, to the dislike of Einar. Kalf invites Einar and his men to vote to exile Lagertha, then turns on them, killing the dissidents and allowing Lagertha to kill Einar. In Paris, Rollo is living as a Frankish noble. He marries Princess Gisla, who weeps through the ceremony and holds a knife to his throat on their wedding night. Rollo rides to the Viking encampment outside Paris, and orders their massacre.