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

2.  quadruplet(n = noun.person) quad - one of four children born at the same time from the same pregnancy;
is a kind of sib, sibling

3.  quadruplet(n = noun.group) quadruple, quartet, quartette - a set of four similar things considered as a unit;
is a kind of set

1.  A collection or combination of four of a kind. [Webster 1913 Suppl.]

2.  Four children born in the same labor. [Webster 1913 Suppl.]

3.  A cycle for carrying four riders, so arranged that all the riders can assist in the propulsion. [Webster 1913 Suppl.]

quadruplet, n.
1 each of four children born at one birth.
2 a set of four things working together.
3 Mus. a group of four notes to be performed in the time of three.

Etymology:
QUADRUPLE, after triplet

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.