The problem with Sherlock isn't just that it's good, it's that the two stars of the show are this close to being the biggest stars in the world. Which means that finding the time in all the various schedules when there would be an opportunity for everyone to get together and film three episodes is harder than aligning the planets with a croquet mallet. It speaks to the affection the cast has for the series, the fans, and the characters that they are willing to go out of their way to make certain that such alignments do happen, even if they are very far apart.
It's with that in mind that Sherlock series 4 has officially been announced! And it's slightly bigger than usual, as the Baker Street Boys has been awarded the coveted BBC Christmas Special for 2015. Production on the special will begin in this coming January, and the following three episode fourth series will film at some point in 2015, with an expected broadcast date of 2016, be it early or late (my guess is that they will follow on from the special, if they can get schedules sorted). This gives Moffat and Gatiss half a year to finish writing the things, and a full calendar year to film them, which should ease the pressure off somewhat.
After the last series ended, I had my own thoughts on what a fourth series might include, going on the Doyle canon, and a fourth episode opens up those opportunities all the more. Of course, it is early days, and there is absolutely no word on what the special might be about, who might appear, and how awesome Mrs. Watson will be throughout.
Via Collider.