Outpost 4 was an outpost constructed a mile deep on an asteroid of almost solid iron by Starfleet in 2160. The outpost's protective shield was made of cast rodinium. This outpost was one of seven Earth Outpost Stations located in Sector Z-6 on the Federation side of the Romulan Neutral Zone.
In 2266, Outpost 4 came under attack by an unknown starship that was later discovered to be Romulan Bird-of-Prey. Although protected by full deflectors, the outpost was hit by a powerful high-energy plasma torpedo that completely blew out the outpost's deflector shield, as well as taking out its phasers and killing its weapons crew. The second attack by the plasma torpedo hit the outpost, unprotected, resulting in the base's disintegration.
During the Neutral Zone Incursion, the location of Outpost 4 was labeled on a map of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", production art)
Between 2266 and 2366, this outpost had been rebuilt. The location of the new Outpost 4 was labeled on a tactical intelligence analysis star chart. This outpost received a warning from Starfleet Command about an alleged Romulan base on Nelvana III. (TNG: "The Defector", okudagram)