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 eight Earth Outpost Stations located in Sector Z-6 along the Federation side of the Romulan Neutral Zone. This zone was a strip of space that acted as a buffer between the Federation and the Romulans in the Alpha Quadrant.
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. (TOS: "Balance of Terror", display graphic)
According to the Star Trek: Star Charts (pg. 66), there were 23 Earth outposts. Outposts 1 to 8 were mentioned in dialog or were seen on the Z-6 Sector map in "Balance of Terror" and Outpost 23 was mentioned in dialog in "Future Imperfect". Outpost 4 was constructed in an area of space that been contested over during the Romulan War Theater of Operations (2156-2160).