A special episode of "Sherlock" focusing on the villian Moriarity will be aired in 2015, which will lead the plot towards the fourth season of the show. On the other hand, "Sherlock" Season 4 release will be in 2017.
The special episode is currently being shot and is expected to air towards the end of 2015, according to TV. This unique episode will wrap up all the loose ends, which surfaced towards the end of Season 3. Towards the end of Season 3, Moriarty hacked all the digital systems to mock Sherlock and this one-off special episode will address this angle of the story.
Even though Moriarty was supposedly killed in the show, the surprising part is that he is still somehow alive, as per the cliffhanger ending of Season 3. The "Imitation Game" actor Benedict Cumberbatch plays Sherlock in the hit series to perfection and he knows the secret behind Moriarity's revival but he is choosing to keep his lips sealed.
"I casually choose to forget in order to rediscover it again a little bit. I like casually forgetting, not least because there's a lot of other traffic between then and now but also because it's more fun because it's slightly more fresh," Cumberbatch said to MTV. He also added, "It's a very good idea, I do remember that detail. I'm just trying not to remember it so I don't blurt it out."
According to Steven Moffat, the filming for the fourth season of the show is yet to start. The tentative start date is next spring, which implies that the show will air in 2017.