It follows season 3. Basically, the minute season 3 ends. The normal episodes are 90 minutes too. So now we have 10 full 90 minutes stories, one 'gay' unaired pilot and a 10 minutes or so special set after season 2.

Season one:
A Study in Pink
The Blind Banker
The Great Game

Season two:
A Scandal in Belgravia
The Hounds of Baskerville
The Reichenbach Fall
Special (10 min) Many Happy Returns

Season three:
The Empty Hearse
The Sign of Three
His Last Vow

Victorian Special (90 min) The Abominable Bride

Season four: Announced for 2017, currently written by Gatiss and Moffat - oh hell!