(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]
of, pertaining to, or designated as a class next below third, as for mailing, shipping, etc.
as fourth-class matter; by fourth-class mail: Send it fourth-class.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.