If you are a Vikings fan, series four is a must watch. It has the usual plots within plots, excellent character portrayals and gratuitous violence. Everything you would expect, with a couple of extra twists thrown in. The season picks up from the assault on Paris and the general background story is about a return visit to the city. But there are lots of other things going on as well.
Initially, Vikings got some bad press for being historically inaccurate, I assume because it is screened on History. But history is always written from the personal bias of the person recording it, in the case of the Norsemen, this was mostly Christian monks who were anything but pro Pagans. Personally I think the show has everything a good story should include and can't wait for season five.