when did season 4 of sherlock come out
1 January 2017
Sherlock (TV series)
The fourth series premiered on 1 January 2017, with "The Six Thatchers". 
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Sherlock (TV series) [SEP] Series 4 (2017)
By October 2013, a fourth series was commissioned, with scripts planned. Moffat told "The Telegraph" in January 2014, "we're all keen to continue", but said it had been difficult to co-ordinate the lead actors' schedules. Filming began on 4 April 2016 at Pinewood Studio Wales, and lasted until 5 August. In May 2016 it was announced that Toby Jones had been cast as a villain from Sherlock Holmes lore. The fourth series premiered on 1 January 2017, with "The Six Thatchers". The second episode "The Lying Detective" aired on 8 January 2017; the last episode "The Final Problem" aired on 15 January 2017.