The upcoming "Sherlock" special episode, "The Abominable Bride," will air this Jan. 1 and will serve as a placeholder for fans as they wait for the official season 4. However, there are now rumblings that the fourth season will not only complete the plot arc from the season 3 finale but it will also serve as the series' end.

Speaking to Variety about the "Doctor Who" series, showrunner Steven Moffat revealed that unlike the "Doctor Who" series, "Sherlock" is by design eventually going to end. 

"Once Benedict Cumberbatch gives up 'Sherlock,' what are we going to do?" he explained in the interview. "We are going to stop, that's what we are going to do. Most shows have a built-in mortality."

Moffat then went on to explain that "Doctor Who" has the innate ability of going on forever because the show eventually reboots itself after a few years, casting a new cast to play the same characters. This is not applicable to "Sherlock" given that the series is not science fiction and mostly revolves around the personal and professional life of its titular character.

Moffat mentioned that the show relies heavily on Cumberbatch being available for the series. This appears to become less likely as the years go on now that the actor is growing in popularity. Cumberbatch was recently seen as the dragon Smaug in "The Hobbit" trilogy of films and is currently filming Marvel's "Doctor Strange."

Cumberbatch's obligations to star in future Marvel films will severely limit his availability for other projects and it is possible that "Sherlock" will be slashed off to make room for bigger projects such as Oscar-potential films. The actor already grabbed a nomination for his performance in "The Imitation Game."

"Sherlock" season 4 will begin production in 2016 and is expected to release either later in the year or by early 2017. Whether it is the final season or not has yet to be seen.