Cardassia IV was the inhabited fourth planet in the Cardassian system. This system was located in the Cardassian sector, which was a region of space in the Alpha Quadrant. The planet was affiliated with the Cardassian Union.
Li Nalas, Borum and several other Bajorans were prisoners at the Hutet labor camp on this planet during the Occupation.
In 2370, Rionoj was given Li Nalas' earring by a maintenance worker on Cardassia IV. Kira Nerys and Miles O'Brien later traveled to the planet to rescue Li after Kira was given the earring by Quark.
While attempting to dupe Post 24, Kira claimed a Gul Marayn on Cardassia IV was waiting for a shipment of Rulot seeds.
Dukat later claimed the Central Command was not aware of the existence of the camp on Cardassia IV. (DS9: "The Homecoming")
According to the script for "Tribunal", the pronunciation for Cardassia was "car-DASS-ee-uh". 
According to startrek.com, Cardassia IV was a M-class