(in the U.S. Postal Service) the class of mail consisting of merchandise weighing one pound or more, including parcel post and all first-, second-, or third-class matter weighing 8 oz. (227 g) or more and not sealed against inspection.
Pronunciation: (fôrth'klas', -kläs', fōrth'-),[key] —adj. of, pertaining to, or designated as a class next below third, as for mailing, shipping, etc.
—adv. as fourth-class matter; by fourth-class mail: Send it fourth-class.