A Starfleet deep space station used by Professor Richard Galen as a transfer point in 2369, later consulted by Captain Janeway and reported closed by the start of the twenty-fifth century.
Deep Space 4 is a Federation station run by Starfleet. It is referred to in dialogue and on charts but never visited by the camera.
In The Chase (2369), Picard's old archaeology mentor Richard Galen leaves the Enterprise-D intending to reach Deep Space 4 and board an Al-Leyan transport there to continue his hunt for an ancient genetic puzzle. In the Voyager episode The Gift, Janeway notes that the station's records held just one entry on the Hansen family, Seven of Nine's parents.
In The Next Generation, the opening episode of Star Trek: Picard's third season, Picard tells Captain Shaw of the Titan-A that an inspection awaits at Deep Space 4. Shaw knows the station has already been shut down, which exposes the ruse.