"Sherlock" series 3 aired last year but it may take some time before series 4 is finally released. Talking with Digital Spy, series showrunner Steven Moffat explained that filming for series 4 won't begin until spring of 2016. With the most optimistic schedule, this means that "Sherlock" could return on TV by January 2017.
The past seasons of "Sherlock" were shown at two-year intervals — 2010, 2012, and 2014. A Christmas mini-episode aired in December 2013 between series 2 and 3, and the next Christmas special, a one shot period-piece, is scheduled for December this year. Thus, it was initially expected that series 4 would air in 2016.
However, the busy schedules of both lead actors Benedict Cumberbatch and Martin Freeman have delayed the production of series 4. Steven Moffat is also busy working on the next season of "Doctor Who," starring Peter Capaldi.
According to a report from Latin Times, series 4 will see the return of many of the show's regulars such as Louise Brealey as Molly Hooper, Amanda Abbington as Mary Watson, and Jonathan Aris as Anderson. It has been hinted that Andrew Scott may return as Moriarty but only in flashback sequences as Moffat confirmed that Moriarty is dead.
However, series 3 ended with videos of Moriarty filling every screen in London and this forces the U.K. Government to call back Sherlock just as he is leaving for permanent exile. It is possible that Moriarty planned his death all along and now there are new consequences for Sherlock and Watson to deal with.
The Latin Times also points out that Irene Adler might be making a return in series 4 and that Mary Watson, who dies in the original stories by author Arthur Conan Doyle, will not see her end this season although Moffat reiterates that anything is still possible.
Lastly, the report also states that writer/actor Mark Gatiss has hinted that "The Red-Headed League" may be adapted in season 4. The story is one of the most popular works by Doyle and the author famously admitted that it was one of his favorite pieces.
