The quartenary structure of proteins is the shape that results from the orderly interaction of the polypeptides of a multisubunit protein.
some proteins are composed of more than one polypeptide.
Each polypeptide is called a subunit. For example, if a protein is composed to two polypeptides, then it has two subunits.
The polypeptides may or may not be different in primary structure. This is dependent upon the nature of the protein.