According to a Digital Spy article dated January 9th, whilst series 4 of Sherlock has not yet been confirmed by the BBC, it is expected to be announced soon. Airing to 8.77 million viewers around the UK, His Last Vow aired last night to great critical acclaim and saw the return of Sherlock’s nemesis, Moriarty saying, “missed me?”
Moffat says he and Gatiss have already planned out series 4 and 5 and, if given the green light by the BBC, will begin filming as soon as Benedict Cumberbatch and Martin Freeman’s schedules allow.
For more information, take a look at the Digital Spy article here: http://www.digitalspy.co.uk/tv/s129/sherlock/news/a542627/sherlock-series-4-and-5-already-plotted-out-says-steven-moffat.html
UPDATE: It appears the BBC has indeed green lighted series 4 as the producers of Sherlock are being pressured to get the next series of Sherlock made in time for Christmas 2014. This is information is coming from the Daily Mail website: http://www.dailymail.co.uk/news/article-2538467/Sherlock-time-Christmas-BBC-bosses-urge-producers-fast-track-new-episodes.html