Steven Moffat shared the news while speaking alongside Sue Vertue at the BAFTA winners press conference, reported Digital Spy. He earlier said the hit BBC show, starring Benedict Cumberbatch as Sherlock Holmes and Martin Freeman as Doctor John Watson, will represent "an emotional upheaval" and leave fans desperate for series five.

"(Series four) is going to be... I suppose you'd say... consequences. It's consequences. Chickens come to roost. "It's dark in some ways - obviously it's great fun and a Sherlock Holmes romp and all that - but there's a sense of things coming back to bite you," he said.

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Before the fourth series hits screens, fans can look forward to a one-off 'Sherlock' Victorian special, which will air later in 2015.

The 90-minute special will be completely separate from other episodes, with Steven Moffat describing it as 'its own little bubble'.

'Sherlock', which began in 2010, aired its third series last year.