"Sherlock" Season 4 is speculated to air in 2016. Though the fans of the show can't wait for the new season, going by the past record it is no surprise that the BBC series would air after a gap of two years. "Sherlock" season 1 aired in 2010, "Sherlock" season 2 aired in 2012 and season 3 aired in 2014.
According to show writer Steven Moffat there is no concrete news about the premiere of Sherlock Season 4.
"I have no idea," Moffat said about the season's premiere in his interview to IGN, adding, "Sue's working on the schedules right now. I just started with Capaldi on the new series of Doctor Who. You know, we'll do what we can, but with no sacrifice in quality -- that's the thing. If people have to wait two years, they'll have to wait two years. When's the next James Bond film? You know, it's not that kind of show that turns up all the time. It's just not. But the good news is, that probably means it will turn up sporadically for a very, very long while."
Moreover, it also being said that the main cast of "Sherlock" Benedict Cumberbatch and Martin Freeman are busy with the movie "The Hobbit." The two have been casted together in the movie and there busy schedules is one of the reasons why there can be delay in "Sherlock" Season 4 premiere date.
Cumberbatch plays the character of Smaug in "The Hobbit" and he talked about his character in his interview to Ryan Lambie of Den of Geek.
"He's just legendary in the world of The Hobbit, and he is the ultimate dragon. He's an extraordinary creation in the book. And what we've all managed to achieve - especially the people at WETA, the post-production and sound facilities in Wellington - is extraordinary. It's another leap forward in cinema, as Gravity was - an environment you can believe in totally, and with the 48 [frames per second] it's extraordinarily real."
Meanwhile, Freeman would be seen playing Bilbo Baggins in the "The Hobbit."