Illustrated Glossary of Organic Chemistry

Tetramer: A structure composed of four identical smaller units, or four very similar smaller units. The units may be associated by covalent bonds or by noncovalent forces.

This tetrapeptide is Gly-Gly-Gly-Gly,  a tetramer composed of four identical amino acid residues (each is a glycine).

This tetrapeptide is Gly-Cys-Ser-Ala, a tetramer composed of four similar (but not identical) amino acid residues.

Lactate dehydrogenase is a tetrameric protein, consisting of four identical protein subunits. The subunits are held together by noncovalent forces.