The six years since BBC's wildly popular modern take on Arthur Conan Doyle's classic detective stories, Sherlock, debuted on the small screen have been an extremely transformative time for the show's stars. Benedict Cumberbatch has received an Oscar nomination for his role in The Imitation Game, played a new version of classic Star Trek villain Khan in Star Trek Into Darkness, and this year will make his debut in the Marvel Cinematic Universe as the titular protagonist of Doctor Strange. Martin Freeman, meanwhile, also joined the MCU in this year's Captain America: Civil War after leading Peter Jackson's Hobbit trilogy as Bilbo Baggins, and had a core role in the first season of FX's Fargo.

While all of this success is good news for the two actors, it was potentially bad news for Sherlock, as the siren call of Hollywood leaves little time for starring in a British TV drama. But thanks to Freeman and Cumberbatch's affection for the show, and for showrunners Steven Moffat and Mark Gatiss, Sherlock's fourth season is set to arrive in early 2017.

As an early tease of things to come, the BBC has released the first promotional still from Sherlock season 4, which shows the dogged detective teaming up with a canine companion. Presumably the bloodhound (who may be a version of Toby, a sniffer dog that Sherlock uses in the Doyle story "The Sign of Four") is just a temporary ally, and not a permanent replacement for Doctor Watson.

Check out the image below:

Sherlock Season 4 - Sherlock with dog

Sherlock's seasons are only three episodes long, but since each episode is 90 minutes they are effectively feature films. The Telegraph spoke to Moffat about the challenges of making a show that features two major Hollywood stars, and the co-creator of the show admitted that it makes it difficult to see far into Sherlock's future.

"I don’t know how long we can keep it going. I’m personally willing but I’m hardly the main draw. I would be moderately surprised if this was the last time we ever made this show. But it absolutely could be...

"We do have two film stars in the programme. They haven’t needed to do these jobs for a very long time. They’re coming back because they want to. I’m amazed that we’ve got this far. I thought that once they had become extremely successful, we would only get to do one more series. There’s never going to come a time when we do a longer run, because this is what the series has become. It’s an occasional treat where you get three movies. It’s how it works."

Given how prolific Doyle's career was, Moffat and Gatiss certainly won't be running short of source material any time soon. The show's formula mixes in characters and plot points from the Sherlock Holmes stories, but mostly reinvents them in order to keep the audience guessing. Toby Jones (Captain America: The First Avenger) is set to play a classic villain in the fourth season, but when asked about his character's identity by the Radio Times he replied, "The only thing I’m allowed to reveal is that I’m indeed a baddie... I think that even, I’d say, I’m really bad.”

Sherlock season 4 is expected to air in early 2017.

Source: The Telegraph