A homotetramer is a protein complex made up of four identical subunits which are associated but not covalently bound. A heterotetramer

Heterotetramer

A heterotetramer is protein containing four non-covalently bound subunits, wherein the subunits are not all identical. A homotetramer contains four identical subunits....

 is a 4-subunit complex where one or more subunits differ.

Examples of homotetramers include: enzymes like beta-glucuronidase (pictured); export factors such as SecB from Escherichia coli

Escherichia coli

Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

and magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

ion transporters such as CorA.