There has been no official word from the BBC or from "Sherlock" showrunner Steven Moffat  regarding the premiere date for season 4 although a recent report from the iDigital Times indicates that the new season won't air until late 2016 or even as late as 2017. 

According to the report, this is because season 4 doesn't hit production until the middle of 2016. Both Benedict Cumberbatch and Martin Freeman have full schedules particularly due to their involvement in several Marvel films such as "Doctor Strange" and "Captain America: Civil War." 

The Christmas Special, however, is still due this December and will be a one-time event where the story takes place in Victoria-era London instead of the modern setting that the show is known for. Contrary to rumors spreading across the Internet, it will be a stand-alone episode and not a time-travel plot where the modern characters travel to 19th century London. 

Co-creator Mark Gatiss recently posted via Twitter a quote from the 1892 Sherlock Holmes story "The Adventure of the Blue Carbuncle" which reads: "...it was a bitter night so we drew on our ulsters and wrapped cravats around our throats."  This quote hints that the Christmas special will focus on this particular case. 

However, some changes were done to the Arthur Conan Doyle story. According to a report from Entertainment Weekly, Moffat explained that the female characters will now have more prominent roles and will have spoken dialogue compared to how they were portrayed in the original story. 

"In the original stories, none of the women really speak. They just don't talk unless it's specifically germane to the plot," Moffat told EW. "So it was a challenge for us in terms of what we're going to do with our female characters now that we're back in Victorian times... which sort of became quite central to how we handled it."