tetrad(n = noun.quantity) 4, four, foursome, iv, little joe, quadruplet, quartet, quatern, quaternary, quaternion, quaternity - the cardinal number that is the sum of three and one;
is a kind of digit, figure

1.  The number four; a collection of four things; a quaternion. [1913 Webster]

2.  A tetravalent or quadrivalent atom or radical; as, carbon is a tetrad. [1913 Webster]

tetrad, n.
1 a group of four.
2 the number four.

Etymology:
Gk tetras -ados (as TETRA-)

N  quaternity, four, tetrad, quartet, quaternion, square, quarter, tetract, tetragon, quadrangle, rectangle, tetrahedron, quadrature, quadrifoil, quadriform, quadruplet, quatrefoil, tetrapod, quadrangle, quad, quadrupole, quadrivium, four, quaternary, quaternal, quadratic, quartile, tetract, tetractic, tetractinal, tetrad, tetragonal, square, quadrate, quaternity, four, tetrad, quartet, quaternion, square, quarter, tetract, tetragon, quadrangle, rectangle, tetrahedron, quadrature, quadrifoil, quadriform, quadruplet, quatrefoil, tetrapod, quadrangle, quad, quadrupole, quadrivium, four, quaternary, quaternal, quadratic, quartile, tetract, tetractic, tetractinal, tetrad, tetragonal, square, quadrate.