The Cardassian Fourth Order was a military unit within the Cardassian Union.
Gul Trepar was the commander of the Fourth Order in 2357 and still held the position in 2373. (DS9 novel: Fearful Symmetry, DS9 episode: "Ties of Blood and Water")
In 2369, Gul Jasad contacted the Fourth Order for their assistance in a Cardassian attack on Deep Space 9. (DS9 episode: "Emissary")
In Grathon Tolar's forged holoprogram of a Dominion discussion on the invasion of Romulan space, the Fourth Order was to assist the Jem'Hadar in the thrust through the Glintara sector. (DS9 episode: "In the Pale Moonlight")
The Cardassian Fourth order had operated covertly in the Glintara sector since the 2350s. (DS9 novel: Hollow Men)