These comments refer to series four as a whole not just the final episode.
Like the previous series this features a single story for the entire six episodes. The story opens with local man Thomas Malone returning to Shetland after twenty three years in prison; he was jailed for the murder of a woman but is conviction has been found to be unsound... most people on Shetland including Drew McColl, the now retired officer who arrested him, are convinced of his guilt. Tensions rise even further when Drew McColl's daughter Sally, a journalist, is murdered. He is certain that Malone killed her as revenge for being put in prison. DI Jimmy Perez and his team investigate and wonder if Sally's murder could be linked to her work. This investigation takes them to Norway where they learn she was looking into dangerous political extremists. These aren't the only suspects though; further digging into the original murder reveal new motives and suspects, including one rather close to DI Perez.
I'm glad that the makers decided to stick to the single story again as the longer stories inevitably give greater detail and better character development. The case is interesting with several suspects emerging over the course of the series. While the way this sort of story works means one wouldn't usually suspect Malone of really being the killer but the way he is played by Stephen Walters makes one think he could be guilty of something and is certainly unstable. The Norwegian angle added to the story and provided an excuse to take the story off Shetland for a while. There are some real surprises as the truth emerges and a tense conclusion. Douglas Henshall continues to impress in the lead role as Perez and Alison O'Donnell is on fine form as Alison 'Tosh' McIntosh, a character still affected by the events of the previous story. The rest of the cast are impressive too. As one would expect there is plenty of stunning Shetland scenery as well as some fine Norwegian views. Overall another fine series of this enjoyable crime drama; I was pleased to hear it will be returning next year.