I have just noticed that there is a special episode of Sherlock called "The Abominable Bride". Does it relate plot-wise to the previous season (3)?
Yes, it is.
Beware, spoilers with regard to the concept of the episode in this excerpt from The Verge's review:
This isn’t a true alternate reality, though: having had his season-ending suicidal mission cancelled, the present-day Sherlock has taken a potent — and potentially lethal — drug cocktail. It’s heightening an elaborate fantasy within his "mind palace," and it revolves around an old case that he’s using to solve the mystery of his nemesis Moriarty’s (Andrew Scott) apparent return.