When we left season 3 behind Rollo stayed in Paris. Due to a prophecy by the seer he needs to stay there. He is going to marry Gisla. At the wedding, we see a proud Rollo. When they have the wedding night we see the first transformation in Rollo. Gisla is afraid of him. The wedding night turns out as a disaster.
After the wedding, Rollo, count Oddo, and Roland make a battle strategy he shows them because he can't speak any French. While Gisla filled for divorce, Rollo starts to learn French. In this moment, we see that he is still a Viking but when he butchered the Viking camp cause he knows it was them or he that got killed when he arrived at the camp, he betrayed Ragnar for the first time this season.
Gisla, The Emperor, and the bishop want Rollo to sign the divorce papers because the marriage is not yet conceived, but when Rollo starts to speak French and beg Gisla to try, Gisla decided to stay with Rollo. Afterwards, Gisla changed but Rollo too. When they conceive their marriage in the next room. While the rest is having dinner because it is Christmas.
After these events, Rollo starts to plan his big battle against his brother Ragnar. When Ragnar shows up for the first attack he defends Paris against Ragnar. He and Gisla are pregnant from their first child. Rollo tries to have sex with Gisla but she thinks it will cost the baby's live so she refuses. Rollo hates the way the Franks live. This is the first time we see Rollo doubt the change he made is any good.
When Ragnar comes for the second time he got a little defeat but we all know Rollo he will attack again. This time, he attacked and wins the battle but with the deadly injured Lagertha the fight between the brothers stopped. Rollo got crowned as Duke and got a "hail Caesar" from the Emperor.