What is another word for "tetrad"?

Roget's Thesaurus (1911) lists these synonyms for "tetrad": animal, area, dimensional, form, four, fundamental, geographical, object, quad, quadrangle, quadrate, quadratic, quadrature, quadrifoil, quadrivium, quadruplet, quarter, quartet, quartile, quaternary, quaternion, quaternity, quatrefoil, rectangle, reduce, square, tetragon, tetragonal, tetrahedron, tetrapod, three, with.