The one-off #Sherlock special will begin filming on January 6 2015, per this new interview with @Markgatiss: http://t.co/OtYANsmEwH
— Sherlockology (@Sherlockology) December 23, 2014
One thing you need to be aware of, in case you didn't know: No, there was NOT a Sherlock special Christmas 2014. You didn't miss anything.
The special hasn't filmed, so it's tentatively slated to air Christmas 2015, which Mark says he and Steven Moffat wrote together. 8) Filming for the rest of s4 will probably happen much later than planned.
So that means s4 setlock will start with filming the 2015 special one-off ep.
I have no idea when the rest of s4 will be filmed. After Ben does Hamlet? Before autumn? Done in parts the way they worked around The Hobbit in 2013? I don't know.
About the rumors on IMDb -- maybe Tom Hiddleston has a part in s4, but I wouldn't bank on it until Sue or Mark confirms. Or Sherlockology.
(Note that both Steven and Mark have mentioned they'd like Tom in SH, as of April 2014, but not that they HAD him.)
Anyone can put stuff on the IMDb. So the rumor does not mean that is true. Wait and see, okay?
And BBC Three has some comments about s3 ep3, His Last Vow. BBC Three is the shipper account of the Beeb, apparently. ;)