The fourth and most expensive of all the Babylon space stations, constructed in Sector 14. Shortly after is completion in 2254, it disappeared without a trace. It reappeared briefly in 2258, at which time Commander Sinclair
led a mission to get the station's crew off.
Babylon 4 was actually sent back through time by the Great Machine, in a mission begun in 2260, to assist the Minbari in their fight against the Shadows. Sinclair traveled with the station back through time.