If this new promotional image is anything to go by things look decidedly frosty between Sherlock and John Watson.
Sherlock series 4 was unlikely to be a cake walk for our intrepid investigators and now, thanks to a gloomy new image, it seems almost certain friendships will be tested, loyalties divided, and something is going to come between Sherlock (Benedict Cumberbatch) and Watson (Martin Freeman).

As is tradition, details on the news series have been scarce but we’ve been given a few one word hints like ‘Thatcher’, ‘Smith’ and ‘Sherrinford’ for the new run of episodes –  the ‘Smith’ part of the equation almost certainly refers to Culverton Smith, a new baddie with a line in exotic poisoning, played by the Dream Lord himself, Toby Jones.
But how dark do things get? Well according to Benedict Cumberbatch, “Myopically dark. You’re talking about the end of the universe darkness. You can’t see in front of you and would walk into everything dark.”
In the same interview with Collider, Steven Moffat adds:
“We turned the lights off to save money, so you can’t really see it. It’s the same show. Hopefully, there are lots of laughs and lots of great personal stuff, but it is explicitly a darker season. You’ll have to wait and see.”
Weighing up the difference between Moriarty and our new villain, Culverton Smith, Mark Gatiss added:
“It’s an interesting thing to chart. We made our Moriarty very different to Doyle’s. He’s Irish, and he brings all his charm, his twinkle and his humor to it while he’s also terrifying. Magnussen was a very blank, chilling business man. He doesn’t see what he’s doing as evil. Toby is doing something very interesting. She’s an avuncular, funny seeming man with terrible teeth. We’ve given him terrible teeth, which are symbolic of the rot inside him. It’s a great complex, shaded character. You’re not quite sure what the relationship is with him.” 
One detail we can also expect to see is Mary (Amanda Abbington) and Watson dealing with life as new parents.
Until these and more details come into focus, why not scratch that Sherlock itch by watching the teaser trailer, just one more time.

Production has wrapped on Sherlock but, as per usual, the waiting continues until Series 4 airs on New Years Day.