For its fourth season, the PBS “Vienna Blood” murder mystery series does something new.

“It’s a cliffhanger! For us, that’s exciting,” said Matthew Beard, 35, the show’s London-born star in a Zoom interview from his home.

Beard is the Freudian psychoanalyst Max Liebermann who in this 1909 setting assists police detective Oskar Rheinhardt (Jüergen Maurer) with his psychological insights into a series of disturbing crimes.

A sub-theme: growing antisemitism against the Liebermann family. Oskar is a lapsed Catholic.

“At the beginning of this season Max has come back from a very successful press tour in America because he’s taken Freud over there and it’s gone down very well (which is, as a lot of this series is, loosely based on how well Freud did when he went there). And he’s also been given a promotion at work.

“So he’s riding high when he comes back. But for him, that side of work is not as thrilling as being with Oskar, working on cases. So he very quickly gets back into that.”

New this season is the shift from weekly odd couple’s crime solving cases to a season-long plot that evokes classic noir.

“For me and Juergen, before any new season we say to the producers, ‘Is there something new? Let’s just not copy the last one.’ ”

As to what people should know who have never seen the series before, “First, I’d say, go watch all the other ones. But if you can’t do that, then know by this point there is this long-developed odd couple relationship between this young psychoanalyst who gets partnered with this policeman.

“In Season 1 anyway, he has a straightforward view of how to solve crime. That involves finding criminals and evidence — the physical side of it.

“I come along with all these theories of the mind, which sounds like nonsense. So we begin at very different ends of the crime-solving spectrum. We’re very different people.

“As the seasons have progressed, this relationship’s got deeper and deeper. We’ve found out more about their personal lives.”

Does this bromance have Freudian implications?

“Everything in the show has an extra Freudian angle!” Beard answered. “That’s why his theories are so exciting and enticing — they can be applied to everything.

“Over the years we’ve definitely enjoyed fans’ reactions and their theories. Especially in the States, actually seeing how a lot of the PBS viewers have responded has always been really funny to me and Juergen, for sure.”

“If you missed the first three seasons,” Maurer, 57, concluded, “you would have seen a fully developed bromance while we’re trying to solve a major crime at the end of the Austrian Empire.”

“Vienna Blood” airs Jan. 5 on PBS