He totally wasn't supposed to say it, but he did anyway. In an interview with RadioTimes Tumblr god Benedict Cumberbatch said that there will, in fact, be another season of "Sherlock" following the third that's about to begin filming soon.
We've agreed to two more [series] but I could get into trouble for saying that...All I know at the moment is we're doing these three [episodes of the upcoming series] and another three. I just don't know what there is beyond that.
But will there be even more after that? Maybe! He added:
It just depends on Martin and I's availability, how long we can keep it going. It depends on Steven's ability. I'd love to keep it going
Martin is of course Martin Freeman, the Watson to Cumberbatch's Sherlock Holmes. If the possibility of more "Sherlock" is based partially on Freeman's availability, further seasons beyond a fourth could be up in the air based on the Martin's commitments to another 52 "Hobbit" movies, in which he plays Bilbo Baggins.
Steven is Steven Moffat, showrunner of both "Sherlock" and "Doctor Who."
Filming on "Sherlock" season 3 begins March 18 with an eye to air later this year. Cumberbatch can soon be seen playing a guy not named Khan but probably named Khan in "Star rek Into Darkness."