a four-piece set is a three-piece product without a label.

"three no products" is not a legal concept, but it has become a popular term, which means no production date, no quality certificate (or production license) and no producer name. There is also a saying that "three no products" are products with no manufacturer name, no production site and no production hygiene license code. It is also said that there is no factory name, no address and no trademark.

"Four-piece suit" usually refers to four sets of bedding in people's daily life. As the name implies, it is a bedding consisting of four sets. The necessary elements are a pair of pillowcases, a quilt cover, a sheet (bed sheet style), a bed hat (bed hat style) or a bed skirt (bed skirt style).

the above content reference: Baidu Encyclopedia-four-piece set.