Four

Four was a Monoid who lived on the Ark in the 57th segment of time, seven hundred years into the vessel's voyage.

Four did not believe One was an effective leader and didn't agree with his decision to colonise Refusis II. He believed the Refusians too dangerous. One, however, ignored Four's fears. Upon arrival on the surface of Refusis, One granted Four and anybody who chose to go with him permission to return to the Ark. One and his supporters remained on Refusis. However, One had no intention of letting Four leave. He had him ambushed on his way back to landing pods. In the resulting firefight, many Monoids died, but Four survived and was eventually taken prisoner by the human Guardians. The Refusians agreed to let the humans colonise the planet, as long as they were willing to let the Monoids remain and peacefully coexist with them. Four was allowed to go free and continue his life on Refusis. (TV: The Ark)

Behind the scenes
Of the Monoid actors (Edmund Coulter, Frank George, Ralph Carrigan, John Caesar), it is never made clear who played which Monoid.