In a recent interview, Sherlock supremo Steven Moffat was asked about rumours that series four of Sherlock might be shown at Christmas. His answer seems to imply it’s unlikely:
We have no idea what we’re going to do. I’m starting the third week of the new [season] of Doctor Who, so I’ve got quite a lot going on. I doubt it. But, it’s a matter of when everyone’s available, and when Mark [Gatiss] and I think we’ve got a strong enough script. We’re quite picky. We want every episode of Sherlock to be really, really good. That’s out ambition. We don’t have to make a million episodes. We have to make some very, very good ones. The moment we drop that idea, it will stop being what it is.
Still, we can hope!