A tetramer is something formed out of four sub-units. The associated propriety is called tetramery.
In chemistry, the term refers to a molecule made of four monomers (for example kobophenol A, a molecule formed from four stilbenoids monomers).
In biochemistry, it similarly refers to a biomolecule formed of four units, that are the same (homotetramer), i.e. as in Concanavalin A or different (heterotetramer), i.e. as in hemoglobin. Hemoglobin has 4 similar sub-units while immunoglobulins have 2 very different sub-units. The different sub-units may have each their own activity, such as binding biotin in avidin tetramers, or have a common biological property, such as the allosteric binding of oxygen in hemoglobin.