While BBC has yet to begin the filming of "Sherlock" season 4, in spring, speculations about the series are starting to appear online.

Recently, Express UK reported speculations about the possible set location of the show for season 4. According to the publication, the scenes for the upcoming season of "Sherlock" could possibly be filmed at the Pinewood Studios in Wales, located on the borders of Cardiff at Wentloog.

The production facility, which is 70,000 square feet in area, is also the place where some of the upcoming films were shot, including the "Assassin's Creed," "Alice Through the Looking Glass," and "Me Before You." It is also the place where "Harry Potter and the Deathly Hallows" was filmed.

The report mentioned that the series has been filmed in South Wales in the past. More recently, the Christmas special episode was filmed in Bristol.

Meanwhile, the plot for the upcoming season remains to be under wraps, aside from the earlier revelations provided by series creators Steven Moffat and Mark Gatiss.

In an interview with Entertainment Weekly, Moffat revealed that the upcoming season will be a little darker compared to the previous seasons.

"We're going to try to take it somewhere we've never been before — and I don't mean outer space," Moffat teased. "It will be a challenging place to go. We always start with a re-introduction that builds toward a darker climax — we might just be going darker [from the outset]. We got a very, very good set of ideas."

He also said that the upcoming installment of the series will be better than the first three.

"We have a plan to top it. And I do think our plan is devastating. We've practically reduced our cast to tears telling them the plan ... we're probably more excited that we've ever been about Sherlock."

There are also speculations saying that the upcoming season could possibly center on the death of Watson's wife Mary Morstan.