Erelyn IV was a M Class planet twelve light-years from the Crab Nebula. The planet was the homeworld of the Erelynian species.

Hundreds of years ago, radiation from the Crab supernova killed all life on the planet. The Erelynians were in the process of developing a type of warp drive that could have helped them learn of the impending supernova, if the politicians of Erelyn IV had not cancelled the program. The buildings of Erelyn IV survived and were later studied in detail by the Vulcans. (DS9 - Millennium novel: The War of the Prophets)