Four

Four was a Monoid who lived on the Ark in the 57th segment of time, 700 years into the vessel's voyage.

Biography
Four did not believe that One was an effective leader, and did not agree with his decision to colonize Refusis II, believing the Refusians to be too dangerous. One, however, ignored Four's complaints. Upon arrival on the surface of Refusis, One granted Four, and anybody who chose to go with him, permission to return to the Ark while One and his supporters remained on Refusis. However, One had no intention of letting Four leave, and 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 colonize the planet, as long as they were willing to let the Monoids remaain as well, and peacefully coexist with them. Four was therefore allowed to go free, and continued his life on Refusis. (DW: 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.