Wiktionary
heterotetramer
n. (context chemistry biochemistry English) A tetramer, especially a biologically active one, derived from two or more different (but similar) monomers
Wikipedia
Heterotetramer
A heterotetramer is protein containing four non-covalently bound subunits, wherein the subunits are not all identical. A homotetramer contains four identical subunits.
Examples include haemoglobin (pictured), the NMDA receptor, some aquaporins, some AMPA receptors, as well as some enzymes.