You know, I suspect it has been confirmed and we’re just all too confused to know it. The first big article that came out on the heels of S3 was this one: http://www.theguardian.com/tv-and-radio/2014/jan/09/sherlock-back-fourth-series-steven-moffat
The way it reads, a lot of people assumed that the headline is referring to Moffat’s mention about planning S4 and 5, at the end of the article. But given that in that article, Moffat also refers to them beginning to work out everybody’s schedules, a subject they’ve since continued to discuss actively in the press and keep fans informed on, I think he really did mean that they’re signed for S4. They wouldn’t be putting this much work into it if it weren’t pretty solid.
I think there’s been a miscommunication somewhere, probably because Moffat likes to backtalk on himself so much and then everybody gets used to not believing his BS for anything short of the literal words “the BBC has confirmed for season 4.” (And it doesn’t help that a lot of news services are confusing the issue further by jumping the gun with declarations like, “Creators say new Sherlock for Christmas 2014!” ”Moriarty is alive!” “Season 5 confirmed!” No, season 5 is plotted.)
But with all this talk of S4 that’s been going around, nobody involved in the show’s production has at any point said anything like, “No no, we’re still waiting for final confirmation.” Including the BBC guy responsible for commissioning these shows.
I suppose it’s possible that the BBC is waiting till they know what sort of scheduling they’ll be dealing with before they give their final commitment. But considering that Sherlock’s domestic viewership per episode hit over 8 million viewers, and 16 million internationally—both the kinds of ratings numbers that TV series aren’t seeing much of these days—for once I’m prepared to take it on faith.
As for it airing in 2016 and not before, there is no confirmation on that, but the warning that it may take that long comes directly from the show’s creators. This is the first mention of it I remember seeing, back in early Feb. And here’s a more recent article, saying the same thing.