'Vikings' season 4 news, spoilers: Ragnar's sons become warriors; hero not dying anytime soon?

The "Vikings" season 4 midseason finale took a giant leap in time, moving far past Ragnar's (Travis Fimmel) defeat to Rollo (Clive Standen) in Paris to a time when his sons had grown into young men ready to take over Kattegat's kingship.
With the massive time jump, "Vikings" fans were introduced to the new actors who will henceforth be playing the roles of Ivar, Ubbe, Sigurd, and Hvitserk.
According to IGN, the casting of actors Alex Høgh Andersen (Ivar), Jordan Patrick Smith (Ubbe), David Lindstron (Sigurd), and Marco Islø (Hvitserk) was kept secret for months. The four young men were unable to tell even their family and friends about what they were doing in Dublin, where History's popular period drama is filmed.
"It's almost a new generation to 'Vikings,'" Smith told the outlet. "Vikings, staying true to the era, is young men becoming warriors."
"To watch four young guys get that jump start that [Ragnar] got, they're pretty much starting from where he left off, and then you get to see them go even further," the actor added. "You need other people to come in to keep the fighting and the life of Vikings alive, rather than having old men chopping each other down."
The introduction of Ragnar's heirs as adults paves the way for increasingly complicated conflicts between Ragnar and the young men, who, having grown up in his shadow, have come to resent him.
In addition, the season 4A finale showed that Ragnar had become some sort of pariah in Kattegat. In it, he announced a challenge, saying that anyone who wants to take the throne is welcome to try and kill him.
Does this mark the beginning of the end of the legendary Viking warrior?
Ragnar fans need not fear, as "Vikings" creator Michael Hirst told The Hollywood Reporter in April that "Ragnar's story is nowhere near the end."
"Vikings" season 4 returns to air its 10-episode back half in the fall.