"Sherlock" executive producers Steven Moffatt and
"It's always special to return to 'Sherlock' but this time it's even more special as we're doing...a special," said Gattis, who co-created the new adventures of Sherlock Holmes along with Moffat.
The "Sherlock" series has been slightly different in format in that each season consists of three feature-length episodes. The fourth season will also be three episodes with the addition of the "special," which will deal with the third-season cliffhanger reappearance of Sherlock's mortal enemy, Moriarty.
Finding time to film each season has been tricky as both Cumberbatch and Freeman are in high demand and Moffat is also the showrunner of "Doctor Who." But filming for the special episode is set to begin in January 2015 and the fourth season going into production later in 2015.
The last episode, "His Last Vow," aired in January 2014, meaning fans will need to wait more than a year for new episodes. However, the wait between the second season and the third season was two years, so long waits are nothing new to the very patient fans of "Sherlock."
Executive producer Sue Vertue said in a statement, "It's taken a little while to get the dates sorted as none of the boys are exactly sitting back twiddling their thumbs."
In the meantime, the actors will be appearing in "The Hobbit: The Battle of the Five Armies," with Freeman starring as Bilbo Baggins and Cumberbatch lending his voice to the dragon, Smaug.