221B still intact at the beginning of ep. 4.3
Sherlock is sitting in his chair in the living room of 221B. And the slate says scene 8-16. So the potential explosion of 221B is at least not the opening scene of ep. 4.3.
So it seems the episode starts in London. Sherlock’s hair looks shorter to me. Maybe it’s to show that he’s fully recovered from whatever happened to him in ep. 4.2. (Look, he’s fine, he shaved and even got a hair cut –> parallel to the beginning of TEH and the end of his undercover work).
The explosion could happen after these first approx. 10 minutes of the episode which makes them take off to the country side. Or they go there for a case and the explosion happens, when they return. (which could be the cliffhanger for the series).