Sherlock has always been a show that has defied conventions. With only three episodes per season at an hour-and-a-half in length each, the modernization of Sir Arthur Conan Doyle's Sherlock Holmes (Benedict Cumberbatch) and James Watson (Martin Freeman) is a bold and defying show. There has been a big hiatus in between seasons, and Season 4 (or, as the British put it, Series 4) has finally began filming after a gap of a few years. Fortunately, fans were sated with the Sherlock movie of The Abominable Bride, which is made to lead right into Season 4 before its official release date.
For those that aren't aware of the show, it is worth watching on Netflix for a good day's worth of binge-watching, and that includes the The Abominable Bride movie release last January. The film was set in Victorian times, at least at first, it then turned out that the Victorian setup was simply a mental simulation from Holmes. It was rather unexpected that the film went that direction, but that is Sherlock for you.
The film definitely dealt with some unfinished business leftover from Season 3. That is, the huge revelation that Professor Moriarty is still alive, or at least has some kind of posthumous back-up plan that involves hacking into screens in London. Part of Sherlock's conflict in The Abominable Bride is trying to figure out how Moriarty could be alive, and this will surely arise in Season 4.
A recent report from Hall of Fame magazine reveals that Toby Jones will be appearing on Season 4. This is something that has been reported on frequently, but his character name was not known at all. It turns out he will be playing Culverton Smith, a man from Doyle's short story called "The Adventure of the Dying Detective". In the story, Culverton is an amateur scientist who has used his knowledge of a tropical disease to kill his nephew. Perhaps he will be the big bad of Season 4, unless Moriarty does show up to cause more chaos.
There is also a report from iDigitalTimes saying that the cast and crew began production in Cardiff, but after the first episode, the filming will begin in Morocco. Some have speculated that filming there is related to some kind of drug hallucination that Holmes could have (as Sherlock's drug problem, which is from the Victorian source material, was addressed in The Abominable Bride).
Perhaps this is where Toby Jones comes in, ready to try and use a deadly tropical disease. Viewers will have to wait until January 2017 at the earliest for Season 4 of Sherlock, and there is more information about some more rumors like Tom Hiddleston and a tragic death here.