First of all let me say the first 3 seasons were good, in fact the first two were just great. There is Viking tradition, there are interesting social and cultural differences between Northmen and others, there is vibrant Viking society and mainly, there is Ragnar; cunning Ragnar, who would "give far more than his eye to acquire knowledge". He is a man of progress, whose logic and wit wins against rigidity and stupidity. He is the man, who comes up with new ideas and surprises everyone who wants to defeat him. Finally, he is the only one capable of carrying this show and pushing it forward.
The third season is also pretty good, although it had slowed down a little bit, dragging the story slowly around, but who cares when Ragnar is going to raid the Paris whatever the cost!
As much I like the first three seasons, there is one thing that kept bothering me throughout second and the third one especially. Every time Ragnar kills some of his enemies (earl Haraldson, king Horik), he creates some kind of vacuum in the society he is not able to fill. Yes, he becomes earl and than king nominally, but he doesn't quite behave like one! I mean as Haraldson had been an earl, he had counselor Svein for the dirty stuff and his guard, he was a judge (in fact), blessed children etc. and all in all there was a lot of things happening around him in Kattegat. Not so with Ragnar; he is just kind of there, not ruling whatsoever, and sometimes it just feels like anarchy there, which I find not entirely plausible. Additionaly, as I have mentioned, there is a feeling of emptiness because of this, emptiness, which I am not quite able to describe. Maybe it is happening, because the second and third series already follows more story lines and can't concentrate on Vikings more, which is in my eyes a threat for this show.
Season 4 takes the good things from its predecessors - throws them away - and builds on flawed stuff of the first three seasons - which is emptiness, story distraction and fade-away from Ragnar. Starting with the most serious issue - Ragnar, it was perhaps the worst decision in the whole show to screw him up. For the entire season 4a, he feels old and tired, doesn't have any desire to explore and raid, doesn't care for his people and children much, and as if it wasn't already too bad, we have to watch him as he becomes a drug addict. This alone is just enough for me. The writers basically take the character majority of viewers cares about most and flush him into toilet. I know he must die eventually, but instead of seeing him suffering for about 9 hours (!!), it would be better to kill him off at his zenith.
Another issue, that hardly appeared in the first three series - there are many meaningless scenes which just kind of fill the time. Consider the whole Wessex feminist thing with Judith - so much time is wasted on her becoming tough and independent, but I don't see a reason why - this is something suitable for 20th century, not 9th. On the other hand there is not much explanation on key events such as why Rollo gave up on his manhood and turned into french nobleman and loving husband so easily.
The emptiness and anarchy I was talking about is really apparent in season 4. It is no more the king Ragnar and his people or the vibrant society from early seasons, it is a mixture of random characters with no depth being around. Typical ruling day of king Ragnar consists of throwing knives, ignoring everyone, doing crazy stuff and taking drugs in the shack, stalking on the rooftop and making a foreplay with that Chinese girl. Björn tries to look as serious as possible, which makes his character so flat it makes me wonder about Ragnar's successor's ability to lead the show (I doubt any of the sons is capable of that). Than the whole thing with Erlendur's revenge is wasted, as everybody knows from the start Björn will not die and Erlendur has even got no plan; when Kalf was still alive, there was a chance to take advantage of it, but now he wants to kill Björn and then will see what's next, probably.
I totally agree with others that it has become a soap opera. Consider king Finehair's arrival. It peaks at the end of an episode, when he basically says in front of everybody he wants to overthrow Ragnar, but that situation is not expanded in the next episode, which is typical for soap operas. Speaking of new characters, they are very uninteresting, and especially Yidu is a complete waste of time, and besides I find presence of a Chinese in the fjord disturbing, she just shouldn't be there; her only purpose is to be a drug dealer anyway. Then there is lot of uninteresting talking about family, children (everybody is with a child now, hearing this gives me an allergic reaction) and how it is being a father, which is just boring. And I can't help myself, but the families look like projections of 21th western family values - different from earlier seasons where we could see more freedom in sexual life and less sensitivity about children and death. Overall the episodes are too talkative and while basically nothing happens in most episodes, everything must have happened in the last one and it created a big mess, but I will not comment on that.
For me, Vikings end with the season 3 and the 4a just does not exist. I am actually reviewing the first season recently and friend I enjoy it as never, because I have found the old Ragnar again and the true nature of Vikings with him.