Four of Twelve, Subjunction of Unimatrix 525 was a drone in the Borg Collective who had the mutation that allowed him to enter Unimatrix Zero.
In 2377, Four of Twelve was summoned to the Unicomplex in Unimatrix 01 by the Borg Queen. She asked him what he remembered about Unimatrix Zero, attempting to locate the carrier frequency that let the drone enter Unimatrix Zero. When Four of Twelve didn't reply, the Queen severed his connection to the hive mind. The drone still said he remembered nothing. The Queen decided to have the drone dismantled and remove his cortical array for analysis instead. (VOY: "Unimatrix Zero")
Four of Twelve was played by actor and art department coordinator Tony Sears, listed as "Borg Drone" in the end credits of the episode.