The writer told The Telegraph that he would be "surprised" if a full fourth run did not follow this year's third series, which concluded last night (November 4) on ITV1.
"I would hope [there will be more] because of the response to this series, but you know [ITV] are always pretty close to their chest in terms of an actual commission," he said. "But I think I'd be surprised if there was not a fourth series."
Fellowes also admitted that he has been "pleased with all" of Downton's series, claiming that the show's second series - which aired in 2011 - was "more solid" than the first.
"We knew who they all were [by series two]," he suggested. "And now we are really involved with them and that takes you forward another step.
He continued: "I am pleased with this [third] series. I thought it worked well.
"It seems to me that the actors have made a very good job of it and the audience response has been great and they kept watching."
Downton Abbey star Brendan Coyle - who plays Bates - recently claimed that he and his fellow cast members know when the show will end, since it has a "finite life".
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Watch a trailer for Downton Abbey's third series below: