Hello there, I do not know whether it is a reliable information but Amanda Abbington IMDB says she play Mary Morstan (not Mary Watson!) only in one episode of S4 which is the Six Thatchers no other episodes are scheduled for her. Interesting, isn't it? Yet

Hey! Yeah, I know! I’ve seen that, too, and wondered… I’m pretty sure AA was filming throughout setlock, though, but I could be very wrong! I mean, I hope very much that Mary dies in S4, preferably unredeemed and canonically awful to her last breath. I hope she goes out fully evil, acting to serve herself as she has all along, with no ambiguity in the slightest for the Mary stans to spin even further than they already twist canon. Like when people comment on a fic of mine and go “wow, Mary is evil in your story!”, to which I generally reply, “like you mean the way she is in canon?” Lol. I just want there to be absolutely not room for wrong interpretations in Mary’s death. I’m rambling, but yeah - maybe they haven’t released the details of the third ep to imdb yet because they’re still sitting on the name? But if the details are out for the second one, I don’t know!! Here’s hoping, though!