One of the asteroid-built Earth outposts guarding the Romulan Neutral Zone, destroyed by a Romulan plasma weapon in 2266 during the incursion shown in Balance of Terror.
Outpost 4 is one of a chain of stations set up along the Neutral Zone after the Earth-Romulan War, each dug deep into an asteroid and protected by rock and deflector screens. In the original series episode Balance of Terror, the Enterprise loses contact with Outposts 2 and 3 and reaches Outpost 4 as it comes under attack.
Its commander, Hansen, reports that an unseen ship has wrecked his defences with a single shot. While he is still on screen the Romulan vessel decloaks and fires again, and the outpost is destroyed.
Strange New Worlds restages these events in A Quality of Mercy, in which Pike lives through a future where he is still captain of the Enterprise in 2266. Outpost 4 is there commanded by Hansen Al-Salah, whose son Maat serves on the Enterprise, and he contacts the ship as the attack begins.