- (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 ounces (227 grams) or more and not sealed against inspection.
- of, relating 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.
- of or relating to mail that is carried at the lowest rate
- by fourth-class mail
