1. We know who is returning for season four - So far, the confirmed cast includes leads Benedict Cumberbatch and Martin Freeman and regulars Amanda Abbington as Mary Watson, Louise Brealey as Molly Hooper and Jonathan Aris as Anderson.
2. Season four will be darker - Moffat spoke with Entertainment Weekly saying the production team wants to take the "Sherlock" series somewhere its never been in previous seasons.
During the "Sherlock" panel at SDCC '15 Moffat talked more about the darker tone of season four saying.
"We've got a clear idea of what we're going to do and it's going to be a clear shift in tone," Moffat said.
3. Irene Adler may return! - iDigital Times reports that Irene (Lara Pulver) may return to "Sherlock" for season four. If you don't remember Irene she was a love interest to Sherlock and much more than that during the early season.
4. Janine Returns! - Another love interest of Sherlock's and Mary's close friend, Janine will return for the upcoming season. According to Moffat she will definitely be back and her and Sherlock will be friends.
5. Moriarty is not Alive! - Moffat said early on that Moriarty is indeed dead and not coming back. The writer said "He shot himself in the face. What more do you want?"
6. Tom Hiddleston may join the cast - The "Avengers" villain is said to be joining the cast of "Sherlock" for season four. Online speculations have said that he will join as the third Holmes brother.
7. Future seasons are already planned! - Moffat told the Guardian that He and Mark already planned ahead for more seasons saying, "Rather excitingly, Mark and I, for no particular reason, we just got out of the rain… and we just started plotting out what we could do in the future." He also added that there was no plan for a movie as each episode is a feature length itself.
8. Will Mary Die? - FashionNStyle reports that season four may see the end of Watson's wife, Mary Morstan. While anything is possible we don't think the creators or producers would get rid of a dynamic female character such as Mary, well not anytime soon anyway. What's more, Martin Freeman has revealed that the show will follow the same path.
9. Steven Moffat reveals season fours theme - "The first series was all about the beginning of their friendship. Second about the formative stages, the love and fear and loss and all that. The third was good days, me and my pal and my pal's wife." Moffat told Entertainment Weekly.
10. Production will begin in the Spring - Chatting with Digital Spy after the Bafta TV awards, show runner Steven Moffat confirmed that series four is planned to start filming in spring 2016.