Quick Answer: During San Diego Comic-Con, the Sherlock cast and crew revealed three important names as clues for Season 4: Thatcher, Smith and Sherrinford. Although we can only speculate as to their true meanings, the names may refer to Margaret Thatcher, the Arthur Conan Doyle antagonist Culverton Smith, and Sherlock's eldest brother, Sherrinford Holmes.
After four days of stars, news and cosplay, this year's San Diego Comic-Con has come to a close. Among the convention's veritable treasure trove of sneak peaks and interviews was a cryptic hint about Sherlock's (2010 - ) forthcoming fourth season. The clue's reveal should be no surprise to fans of the BBC drama; before every season of Sherlock, creators Mark Gatiss and Steven Moffat have released three words to hint at possible storylines and characters in the upcoming season. For Season 2, it was "Woman, Hound, Fall." For Season 3, it was "Rat, Wedding, Bow." Instead of words this time, the Season 4 clues are three names: Thatcher, Smith and Sherrinford.
The cast of Sherlock reveals the three clues for Season 4
The most obvious candidate for Thatcher's identity is, of course, former British Prime Minister Margaret Thatcher. Thatcher was mentioned in the Season 2 episode "The Hounds of Baskerville" when Sherlock (Benedict Cumberbatch) deduced that the project H.O.U.N.D password was "Maggie." John Watson (Martin Freeman) also wrote an old blog post titled "The Six Thatchers." In the blog post, Sherlock solves the murder of an art student that involved the burglary of several pottery figures of Margaret Thatcher. However, since both the storylines in "The Hounds of Baskerville" and the blog post are presumably over, it's not yet clear how Thatcher will be included in Season 4.
Margaret Thatcher was the password for Project H.O.U.N.D.
Smith, although a common name, most likely refers to Culverton Smith, the antagonist of the Arthur Conan Doyle short story "The Adventures of the Dying Detective." In the short story, Sherlock Holmes feigns illness to trick Culverton Smith into confessing to his nephew's murder. It has been revealed that actor Toby Jones will be the main antagonist of Sherlock Season 4 and portray an updated version of Culverton Smith.
Toby Jones as Culverton Smith
The last clue is the name Sherrinford. Arthur Conan Doyle originally considered using the name Sherrinford Holmes before deciding on Sherlock Holmes for his detective stories. However, in 1962, author William S. Baring-Gould wrote a fictional biography for Sherlock Holmes in which Sherrinford is the eldest brother of Sherlock and Mycroft Holmes. In the Season 3 finale, Mycroft (Mark Gatiss) mentions the existence of a third Holmes brother, but not by name. Although Sherrinford is not a canon character from Doyle's original short stories, it seems very likely that Sherrinford will be the third Holmes brother in the BBC series.
Mycroft mentions the existence of a third Holmes brother
The new trailer for Sherlock Season 4 has more new information and even raises several questions. Is Moriarty back from the dead? What is Mary Watson's past? Production for Season 4 is almost done, but we'll have to wait until Sherlock returns in 2017 to learn the answers.