BBC's insanely popular Sherlock series is set to return for its fourth series in 2016.

However, the season 4 release date may be slightly delayed, due to the actors' demanding schedules.

Both Benedict Cumberbatch and Martin Freeman are in such demand that filming of the next series would have to be delayed.

The hit series has already received the green light for season 4 and season 5, and viewers should expect to see Jim Moriarty return in 2015.

The show is also likely to see "increased roles for female characters Molly Hooper (Louise Brealey) and Mrs. Hudson (Una Stubbs)," according to The Hollywood Reporter.

Ealier this month, Sherlock creators Steven Moffat and Mark Gatiss teased fans on the upcoming season 4 of the hit detective drama.

According to The Hollywood Reporter, both creators said that Jim Moriarty, who was shown in the season 3 finale, will be featured in the new season. THR also reported that "the show is likely to see increased roles for female characters Molly Hooper (Louise Brealey) and Mrs. Hudson (Una Stubbs)."

Moffat and Gatiss reportedly provided clues on what is to come for Benedict Cumberbatch's Sherlock Holmes and Martin Freeman's John Watson characters, and also promised that new characters will be added.

After a long wait of two years, Sherlock season 3 finally aired in January this year. The rumored air date for Sherlock season 4 is set to be for 2016, which will be another two years for avid Sherlock fans.

Gatiss recently told Metro: "There was suddenly a kind of outraged response that it might not be back until 2016 but that's precisely how long it always is. It's always two years! But we'd like to return soon, of course." He continued, "The idea of growing older with it would be great. But it is hard to get everyone back together, because they have become extremely famous -- except for us."

Sherlock is a British television crime drama with a contemporary update to Sir Arthur Conan Doyle's Sherlock Holmes detective stories. The show was created by Steven Moffat and Mark Gatiss, and stars Benedict Cumberbatch as Sherlock Holmes and Martin Freeman as Doctor John Watson. Six episodes have been produced so far. The first three episodes aired in 2010, and series 2 aired in 2012. Production for series 3 started in March 2013 before airing on Jan. 1, 2014.