The first half of the season explores Ragnar's internal and external scars following the defeat in Paris. His struggle with leadership, addiction to Yidu's "medicine," and his ultimate loss of faith drive the emotional core of the season.
Season 4 was a major turning point for the show, doubling its episode count to 20 and shifting from the personal saga of to the rise of his sons.
The latter half introduces Ragnar’s grown sons—Ubbe, Hvitserk, Sigurd, and most notably, Ivar the Boneless . Ivar quickly establishes himself as a ruthless and brilliant military strategist, filling the narrative void left by his father.