The series, which is produced by Hartswood Films for BBC One, stars Benedict Cumberbatch as Holmes and Martin Freeman as Watson and has been a solid ratings winner for the channel.
Today channel bosses confirmed that the show would return for a special, followed by a series of three new episodes.
Co-creator, writer and executive producer Steven Moffat said: “A special, plus a new series of three episodes – it’s a record-breaking run!
“Of course, it’s far too early to say what’s coming, but we’re reasonably confident that the very next thing to happen to Sherlock and John, is the very last thing you’d expect…”
Fellow co-creator Mark Gatiss, added: “Series Four! At last ! It’s always special to return to Sherlock but this time it’s even more special as we’re doing… a Special!
“Not only THAT, we’ll then be shooting three more episodes which will take Sherlock and John Watson into deeper and darker water than ever before. The whole Sherlock team are terrifically excited to return for four new adventures. The Game is on – again!”
Charlotte Moore, Controller of BBC One, says: “It’s a very good day in the life of a BBC One Controller when you can announce Sherlock will return for a special plus a new three-part series.
“Massive thanks to Sue Vertue and Hartswood Films for pulling this off, it’s no mean feat pinning down some of this country’s most brilliant and in-demand actors and writers.”