adj (Philately) of or relating to mail that is carried at the lowest rateadv (Philately) by fourth-class mail
fourth′ class′
n. (in the U.S. Postal Service) the class of mail consisting of merchandise weighing one pound or more and not sealed against inspection. [1860–65, Amer.]
fourth′-class′
adj. 1. of, pertaining to, or designated as a class next below third. adv. 2. by fourth-class mail. [1860–65]