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Tom Hiddleston starred in acclaimed BBC series The Night Manager John Phillips / getty

Sherlock season 4 is underway and has already served a major blow by killing Mary Watson played by Amanda Abbington in episode 1 titled The Six Thatchers. As the season unfolds, Sherlock fans investigate every clue dropped by the actors and during the episodes to predict the future of their beloved detective series.

One of the favourite rumours that fans are convinced about in season 4 of the BBC series is Tom Hiddleston's cameo as the hypothetical third Holmes brother Sherrinford alongside British heartthrob Benedict Cumberbatch.

Hiddleston's name was first linked with the popular series when Mark Gatiss – who plays Sherlock's elder brother Mycroft – shared a photo with him and co-star Abbington during San Diego Comic-Con.

Gatiss did not add to the Hiddleston speculations, but insisted that he would be willing to go the extra mile to get him on the show, as per reports.

Meanwhile, the Sherrinford reference received another hint during the season premiere of Sherlock on New Year's Day. In one of the final scenes in The Six Thatchers, Mycroft returns home to find a Post-it note reading "13th" in bold letters. He then makes a phone call and says "put me through to Sherrinford please." Mycroft's reference to Sherrinford led to fans speculating about the possibility of a third Holmes brother.

Author Arthur Conan Doyle had originally intended to name Sherlock as Sherrinford. When Mycroft mentioned "the other one" during a conversation in season three, fans speculated that Hiddleston could be the one playing the anticipated Sherrinford.

"I'm not given to outbursts of brotherly compassion. You know what happened to the other one," he said alluding to the possible existence of another brother. However, the 35-year-old actor himself hasn't confirmed the news of his cameo in Sherlock season 4.

Apart from Sherrinford, the other mystery remains whether or not Andrew Scott will return to the series as the Moriarty.

Sherlock season 4 episode 2 titled The Lying Detective airs on Sunday, 8 January.