Sherlock will still have his English accent, he’ll still solve mysteries without any mysticism, and he’ll presumably do so without a long flowing cloak a la Doctor Strange.
Benedict Cumberbatch apparently switched over to his Doctor Strange accent during filming for the Sherlock premiere, but he kept it behind the scenes.
In fact, it sounds like the biggest change will simply be that Sherlock has become more humanized for Season 4, and Cumberbatch certainly won’t have borrowed that from his performance in Doctor Strange.
Fans don’t need to worry that Sherlock will suddenly sound American when he returns to TV with “The Six Thatchers.”