of or relating to mail that is carried at the lowest rate
adverb
2.
by fourth-class mail
fourth-class in American English
(ˈfɔrθˈklæs)
adjective
1.
of the class, rank, excellence, etc. next below the third
2. US
designating or of a class of mail consisting of merchandise, printed matter, mailable live animals, and all other material not included in first-class, second-class, or third-class; parcel post