of the class, rank, excellence, etc. next below the third
☆ 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
adverb
as or by fourth-class mail; parcel post
See fourth-class in American Heritage Dictionary 4
adjective
Of or being a class of mail consisting of merchandise and some printed matter weighing over eight ounces and not sealed against inspection.