Deep Space 4 was a Federation space station utilized during the 24th century.
In his search for the ancient humanoids DNA-puzzle, Richard Galen wanted to get to Deep Space 4 on a Vulcan transport, and there gain passage to Caere on an Al-Leyan transport. (TNG: "The Chase")
The computer records from DS4 contained information on the Hansen family. Captain Janeway on the USS Voyager, while searching for information on Seven of Nine, could only find the information she needed from Starfleet's database for Deep Space 4. (VOY: "The Gift")