"Sherlock" still has another full year before it returns to the air and fans will get a sneak peek into the new season in a special episode this December. Since the end of season three fans have speculated what's to come in the next chapter of "Sherlock" and more than a few big rumors have hit the interweb. One of the biggest rumors around is that a big guest star will appear in the series as the eldest brother to Sherlock and Mycroft.  

Although executive producer Mark Gatiss is puzzled by the rumors it appears that Tom Hiddleston (from "Thor 3: Ragnarok" and "Avengers: Age of Ultron") could be joining the cast as Sherlock's older brother. In a recent interview Cumberbatch hinted that fans will see more of the Sherlock's family background in the new season. "You find out about Sherlock's background. You find out that he comes from a truly stab family," he said. 

"Sherlock" has traditionally aired a new season every two years, and since the latest season was in 2014, it makes sense that season four will arrive in 2016, although it was reported that it could be pushed back to the 2017 due to the star's busy schedules. However, recent reports have showed that both Benedict Cumberbatch and Martin Freeman were seen filming what appears to be the season four Christmas special in England.

1. We know who is returning for season four - So far, the confirmed cast includes leads Cumberbatch and Freeman and regulars Amanda Abbington as Mary Watson, Louise Brealey as Molly Hooper and Jonathan Aris as Anderson. We can expect more of the regulars to turn up as filming progresses, including the likes of Una Stubbs and Rupert Graves.

2. Key information on the 2015 special episode - The script for the 2015 "Sherlock" special has been co-written by Mark Gatiss and Steven Moffat, with Douglas Mackinnon ("Doctor Who," "Line Of Duty," "Jekyll") directing the episode. Production Designer Arwel Jones is back, while Suzie Lavelle ("Doctor Who," "Ripper Street") joins the team as the Director of Photography.

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. Pulver spoke of returning with RadioTimes saying: 

"Sherlock is Mark Gatiss and Steven Moffat’s passion project. If it fits to have her back, I know they’ll write it; if it doesn’t, I trust them to get it right."

Adler was a dynamic character and her return would be very interesting no matter what the plot is for the fourth 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. The writer told IGN

"He likes her, though. He'll see her again. They'll be pals. I loved Yasmine's performance as Janine -- she still likes him in the end. At some point, you know, if Sherlock Holmes has to go to a function and he needs a date, he'll phone her up. Now, he'll not expect her to say no. She'll be very cross with him, but she'll show up, and they'll have a good time. They will, because he likes her."

5. 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 sat at the top of the [Sherlock] production bus … 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.