From the Financial Times (“‘Sherlock’ a big winner for BBC’s finances”), this quote from Ben Stephenson, controller of BBC Drama:
' “This is not hyperbole,” added Mr Stephenson, who confirmed that the BBC will film a “special with a big twist” in January, followed by a three-part series filmed in 2015, with no fixed date for broadcast.
' “Steven sat down with Martin [Freeman] and Benedict [Cumberbatch] a month ago and took them through what the plans are [for series four]. It's impossible to guess what's going to happen. They are being very bold and brave. There are twists and turns, as you would expect. But it is shocking.” '
Saying that the actors/writers are being “bold and brave” in discussing series 4 really signals something beyond the usual in today’s world of drama (eg Fargo, American Horror Story etc). Bets are on something pretty emotionally difficult with the baby or heading towards a gay relationship.