"Sherlock" Season 4 spoilers indicate that Sherlock will respect his nemesis Moriarty as they both have a great intellectual capacity. Mycroft will spy on his brother because he is worried for Sherlock's safety.
There is doubt whether the much-dreaded villain Moriarty is actually dead or not, as he somehow manages to resurrect himself after being shot. Actor Benedict Cumberbatch, who plays Sherlock, revealed in an interview with Collider that he has a lot of respect for Moriarty despite the fact that they are on different sides of the law. He also added that their intellect and mental prowess are almost at par, which makes it for an exciting fight between the good and the bad.
Meanwhile, "Sherlock" Season 4 is said to be the darkest season of the series. Amanda Abbington, who plays Mary Watson, revealed during an interview with Comic Book that she was depressed a lot during the shoot. She also added that she cried a lot and was amazed by the emotional depth of the stellar script.
Abbington also revealed that despite the darkness and the heavy emotional factor in "Sherlock" Season 4, the humor is intact and it manages to lift the show at all the right moments. She also added that the funny gags will always be an inherent and essential aspect of the show.
At the end of "Sherlock" Season 3, Abbington was shown to be pregnant with John Watson's baby. Hence, it is expected that she may give birth in the pilot episode of "Sherlock" Season 4.
On the other hand, there are also speculations that Abbington may be killed in the show. She may die giving birth or may be killed at the hands of the enemies.
There are very few chances of "Sherlock" Season 5, as the main stars of the show Cumberbatch and Martin Freeman are more involved with their Hollywood careers and do not have time for the TV show. The show's producers and creators want to continue but in lieu of the unavailability of the main protagonists, "Sherlock" Season 5 seems highly unlikely.
Watch the official teaser of "Sherlock" Season 4 here: