"Sherlock" is gearing up for its fourth season as show creators Steven Moffat and Mark Gatiss are writing fresh episodes for the series. While a premiere date has yet to be announced, filming is expected to begin in spring 2016.

Fans have been waiting for a new season since the series aired its season 3 finale in January 2014. While the show is certainly one of the most popular British exports to hit TV screens in recent times, it also has the fewest episodes as it only aired nine episodes since it premiered in 2010.

According to Moviepilot, it is no secret that the reason why there's a large gap between seasons is due to the busy schedules of those involved in the series, particularly Benedict Cumberbatch and Martin Freeman.

Both actors' movie careers have skyrocketed over the recent years. Freeman filmed three installments of "The Hobbit" movies between 2012 to 2014, and apart from other films, the actor is set to make his Marvel Cinematic Universe debut in "Captain America: Civil War" next year.

Meanwhile, Cumberbatch has been in 10 films since 2011, including "Star Trek: Into the Darkness" and he is currently filming Marvel's "Doctor Strange," which is set to premiere in November 2016. Cumberbatch is playing the role of the titular hero and is expected to reprise the role in future Marvel movies.

With the stars both occupied with filming new movies, fans have expressed their worries that the actors—especially Cumberbatch—may not have enough time to film more episodes of "Sherlock."

Moviepilot reports that Moffat has addressed this issue as the show creator said, "Once Benedict Cumberbatch gives up 'Sherlock,' what are we going to do? We are going to stop, that's what we are going to do. Most shows have a built-in mortality."

While the showrunners have not announced a series finale for "Sherlock," it looks like fans will have to be prepared for whatever happens to the series. For now, there is still the "Sherlock" special to look forward to, and the Victorian episode will air on Friday, Jan. 1, 2016 on BBC and PBS Masterpiece.

Season 4 of "Sherlock" is expected to air in 2017.