A trailer for the fourth season of 'Sherlock' has been re-released, this time featuring the air date of the first episode 'The Six Thatchers'. There's no new footage available, but creators were more than generous with what they released earlier this year. Fan theories are rife.

Benedict CumberbatchBenedict Cumberbatch returns as genius sleuth in 'Sherlock' season 4

The 'new trailer' that everyone's going on about is exactly the same as the initial teaser we saw from San Diego Comic-Con, only this time we get to see the release date. It confirms the heavy amount of speculation that it will air on New Year's Day, just like the 2016 special 'The Abominable Bride'.

Moriarty is, at least, still haunting Sherlock (Benedict Cumberbatch), but there seems to be something else going on to. Whatever it is, it's seriously rattled him; in fact, there's a possibility we've never seen the emotionless sleuth so terrified. Could it be the villainous Culverton Smith (Toby Jones) who has struck terror into his heart? If the trailer is anything to go by, probably not. 

Fans of the Sir Arthur Conan Doyle book series will know that Culverton Smith is an antagonist from 'The Adventure of the Dying Detective' from the series 'His Last Bow'. It sees Sherlock Holmes become apparently infected with a deadly disease from an artifact posted to him by Smith. He asks a concerned Watson to bring Smith to him, whereby he reveals his illness was faked and immediately has him arrested for attempted murder. Turns out Sherlock is smart enough to take care when opening unsolicited mail. The question is, could this be the answer to those eerie nurses in the trailer, Sherlock shaking with fear in what looks like a mortuary and Watson (Martin Freeman) sitting at the foot of a hospital bed?

Or is Mycroft (Mark Gatiss) the enemy this time? The good-natured 221B Baker Street landlady Mrs. Hudson (Una Stubbs) can be seen telling Sherlock's brother, 'Get out of my house, you reptile!' Quite out of character, especially when you consider that there's no real animosity between the siblings deep down. 

More: Sherlock season 4 starts filming 

On the other hand, could this exchange have something to do with that clue that Mark Gatiss, Amanda Abbington and Benedict Cumberbatch dropped on the Comic-Con red carpet? They were photographed each holding up a sign reading Thatcher, Smith and Sherrinford respectively. Now we know that 'Smith' probably refers to Culverton Smith, while Sherrinford is well known in Sherlock Holmes fan circles as the hypothetical elder brother of Sherlock and Mycroft - could it have been Sherrinford that Mrs. Hudson was talking to?

'Thatcher' was the real mystery until the title of the first episode was revealed. And if you want a clue about what to expect, visit John Watson's blog post on 'The Six Thatchers'. Just in case it's a huge spoiler, we won't say anything more about it here.

'Sherlock' season 4 comes to BBC One on January 1st 2017.