Fourth-class mail is an obsolete term that referred to the type of mail being sent. Fourth class has been renamed into different categories, such as special standard or library mail.
Any item mailed with a fourth-class mail category is not allowed to be over 70 pounds, no less than 1 pound and no longer than 108 inches. This is also known as parcel post. There is generally no priority for the mail sent under this classification. The majority of items mailed under fourth-class mail are books, merchandise and other printed materials. Many catalogue companies mail their items under this classification.