Four; having four.
Greek, from tettares, four.
This form is widely used. Some examples are tetrathlon (Greek athlon contest), in which each contestant competes in four events; tetraplegic, a person paralysed in all four limbs; tetrameter (Greek metron, measure), a verse of four measures; and tetrad, a group or set of four.
It is common in scientific contexts, for example in tetrahedron, a solid having four plane triangular faces; tetrapod (Greek pous, pod‑, foot), a four-footed animal; tetrapterous (Greek pteron, wing), of an insect that has two pairs of wings.
In chemistry it indicates the presence of four atoms or groups of a particular kind: tetroxide, an oxide containing four atoms of oxygen in its molecule or empirical formula; tetracycline, any of a group of antibiotics with a molecular structure containing four rings; tetrafluoroethylene, a dense gas which is polymerized to make plastics; tetrahydrocannabinol, a crystalline compound that is the main active ingredient of cannabis.
The form tetarto‑ (from Greek tetartos, fourth) now only occurs with any frequency in tetartohedral, a term in crystallography for a crystal that has only a quarter of the number of faces required for full symmetry.