I watched this final episode of season 4 last night and frankly found it disappointing. You can see it online in the US here until January 29. Unfortunately, the writers have once again succumbed to the temptation to go in for surprise plot twists at the expense of plausibility, which was also the big problem with their Christmas special The Abominable Bride from a year ago.
The penultimate episode last week ended with an implausible plot surprise and I thought that it would resolve itself into something believable in the final episode. But instead the writers cranked the preposterousness up to to 11, creating new plot holes all over the place. (One example is the relationship between Eurus and Culverton Smith.) Furthermore, I have said before that I have little sympathy for the cliché of the evil arch-villain who lacks any human feeling and seems to be all-knowing and all-powerful, able to manipulate events remotely. This episode created an even more unrealistic arch-arch-villain.
As a result, I became so disengaged from the story that during the middle section of the program where Sherlock, Mycroft, and Watson have to urgently solve a series of problems in order to save a child who seems to be the only sentient person on a plane, I was actually bored and waiting for them to get it over with so that we could get to the denouement. When it did come, though, it seemed very rushed and unsatisfying. Given that the entire series is designed to keep viewers on the edge of their seats with excitement, my reaction was not a good thing.
Like most of the episodes, there are allusions to the original stories but in this one, the connection to The Musgrave Ritual seemed highly forced, tacked on to the end in an artificial way.
There are mixed signals as to whether the series will return or not. There were no cliffhangers that would signify a definite commitment to produce new episodes and the ending was such that this episode could serve as a closing bookend to the show. But if it does return, the writers would do well to go back to the model of the very first episode in the first season and ditch Moriarty-like super-villains.