Well, that was dark. I was expecting a bit more humor. Perhaps not as much as in The Empty Hearse, but still... Maybe I missed some while switching streams or saw it all in the trailers. I think the dark atmosphere was overdone, particularly the slowmo voiceover at the very end.
The overall mystery was good. I liked the pearl as the red herring leading to the A.G.R.A. stick reveal. I want to go back and watch Sherlock's twitter cases so I can better tell what was going on.
I am glad we ended with likable Mary. I know she was controversial in series 3, but I really liked her in the 3.1 and 3.2. I find it slightly easier to believe John would want to cheat on Mary than that she (or Sherlock!) wouldn't find out about it. I assume the reason John is being a dick to Sherlock at the end is out of guilt.
I thought that Mycroft was going to get shot at the end rather than Sherlock. Could Mary just have pushed him out of the way? It would take more force than just moving herself but then maybe neither of them would have fatally wounded.
What was with that clip of Mary telling Sherlock to go to hell? I don't remember if that was in the trailer for 4.2 or at the end of 4.1. I'll have to go back and check.
So, Sherrinford?