And not just for a Christmas special as previously rumored but for a full (well three episodes) season as well.
Shooting will start in early 2015 for the Christmas special which suggests that will air first, so season 4 itself may not actually air until 2016.
Which is a fair way away certainly. But by Sherlock standards a commitment to 4 episodes within the next 2 years is actually pretty good!
There’s no hints yet about the storylines, although Martin Freeman has previously indicated that his wife Amanda Abbington would be back as well as Benedict Cumberbatch (and presumably Ruper Graves as Lestrade and Louise Brealey as Molly). I think we can also safely assume that the Christmas special will address the return of Andrew Scott’s Moriarty.