Tetra is a Greek prefix meaning 4 (in constrast to quad- as in quadruped, which is Latin). Examples of "tetra" words include:
In Solid Geometry a Tetrahedron (plural Tetrahedra) is a pyramid shape with six straight edges and four trangular faces.
In chemistry, the number of subunits of like kind attached to a central molecule, as in carbon tetrachloride, a central carbon molecule with four chlorine atoms attached.