The downside of Benedict Cumberbatch and Martin Freeman becoming so popular, is that it becomes that much more difficult to get them back together to shoot another season of Sherlock. However, as a way to make up for yet another two year break between seasons of the modernized take on the Sherlock Holmes property, Freeman has indicated that a Sherlock "one-shot" will be released - and more recently, he clarified that said one-off episode is likely to be a Christmas Special, arriving next year.
Sherlock co-creators Steven Moffat and Mark Gatiss are busy putting the finishing touches on Doctor Who season 8 right now, as Freeman and Cumberbatch continue to work on various movie projects, make television show appearances and/or act on the stage. Freeman informed The Telegraph that the band getting back together for a bit of Sherlock filming in early 2015 "looks pretty likely," but cautioned that production on season 4 will probably begin at a later time.
As mentioned before, if Freeman and company do find an opening in their collective schedules early next year, they'll likely use that time to churn out a Sherlock one-off for a Holiday 2015 release. To quote Freeman:
“If that’s going to be a special - I’m speaking off-message here; if this was New Labour I’d get fired - I think that might be for next Christmas. A Christmas special. That’s what I understand."
The Sherlock season 3 finale, "His Last Vow", left the door wide-open for season 4, though it sounds as though the proposed Christmas special is unlikely to focus heavily on over-arching narrative developments for the show. Not to say that's a problem, mind you; big chunks of season 3 were as much about the show's characters as they were the larger mythology of the Sherlock universe, in no small because Sherlock, John, and their peers (Mycroft, Molly, etc.) are simply a delight to spend time with, no matter what they're doing. A one-off adventure with them, as such, would be welcomed by many a fan of the series.
Freeman, as you would've expected, mostly kept mum on any potential major developments in the series' future, while he talked about the prospective Christmas special. However, he did note that although Moffat and Gatiss play "fast and loose with the original stories," Sherlock generally "follows the trajectory" of the Sir Arthur Conan Doyle novels that inspired it. That's not exactly the best news for certain characters on the show - specifically, Mary Morsten (played by Freeman's real-world partner, Amanda Abbington).
The writing isn't on the wall, though, so best to hold off jumping to conclusions about what lies ahead, for our intrepid sleuth and his companions in the season(s) ahead. In the meantime, it sounds as though Sherlock's principal players are close to locking in the dates for continuing their work on their series; hopefully, we'll get a significant update (or two or three or more), before 2014 finishes its run.
Sherlock season 4 isn't expected to arrive before 2016.
Source: The Telegraph