Two of the three episode titles for Season 4 have been released (
you can find them here, along with what they might be based off of) so I have added the titles and 'The Abominable Bride' into the tags. I'll add the third one as soon as it is released.
Also, according to the article in the link, Benedict is signed on for Season 5, so it looks like we'll be getting another season in the next decade. :P