Jack, who is played by 'Death In Paradise' actor Gary Carr, is the first black character in Julian Fellowes' Emmy and Golden Globe award-winning drama.
He was photographed filming scenes in a rowing boat with Lily James, who plays Lady Rose MacClare, the rebellious great-niece of Dame Maggie Smith’s character Dowager Countess Violet Crawley.
Could this be the start of a new love story for the show? Jack's character has been described as "a charming and charismatic young man".
Carr will be joining new cast members Paul Giamatti, Dame Kiri Te Kanawa, Tom Cullen, Julian Ovenden, Nigel Harman, Joanna David and Dame Harriet Walter who are all set to star in the fourth series.
Carnival Films’ Managing Director, Gareth Neame, said in a statement: “We are delighted to introduce another fantastic, dynamic character to Downton Abbey. His addition will bring interesting twists to the drama which we can’t wait for viewers to see in Series Four.”
As well as filming an eight-part fourth series, 'Downton Abbey' will return at Christmas 2013 with another feature-length episode. The new series will see the return of Shirley MacLaine as Martha Levinson alongside series regulars Hugh Bonneville, Maggie Smith, Michelle Dockery and Jim Carter.