Nobody’s saying anything for certain yet. They’re just warning people that scheduling for this show is a complicated issue and it’s perfectly possible it could take that long.
And they’ve been saying that since about a week after S3 finished airing. But that italics right there isn’t me expressing frustration at you, anon. it’s not the fans’ fault, because there are all kinds of conflicting news articles and reports flying around about S4.
I blame The Daily Mail for the scheduling confusion. Somebody in the first few weeks of the year made a comment about how the BBC would love to have something Sherlock for Christmas, and the Daily Mail spun it up into a whole article about how “Sherlock set to return for Christmas 2014!” and confused the pants off everybody. But only a few days after that, the creative team was like, “You guys all know it could take way longer, right? Gosh golly, Christmas 2014 would be awesome, but we can’t make any promises at this point.”
Lately they seem to have upgraded (downgraded?) to “We’d love to be able to do it faster, but it’s looking quite likely it could take two years again.”