He joins Keanu Reeves and Carrie-Anne Moss, who will be reprising their roles in the film, as well as Yahya Abdul-Mateen II, who was tapped as one of the leads last week, reports variety.com.
Plot details are currently unknown.
While it was rumoured a young Morpheus could appear in the movie, sources close to the film would not confirm Harris' specific role.
Warner Bros. announced in August that a fourth "Matrix" movie is officially in the works, with Reeves and Moss returning and Lana Wachowski on board to write and direct.
Aleksandar Hemon and David Mitchell co-wrote the script with Wachowski, who will also produce with Grant Hill. The film is expected to begin production at the beginning of 2020.
The role is a huge land for the "How I Met Your Mother" star in what would mark his first tentpole at a major studio. Harris was most recently seen in the Netflix series "A Series of Unfortunate Events".
On the film side his recent films have included Alexander Payne's "Downsizing" and David Fincher's "Gone Girl."
Jada Pinkett Smith is in negotiations to reprise her role as Niobe in "The Matrix 4". She played the role in the previous three films.
( With inputs from IANS )