The Collaborative International Dictionary of English v.0.48:

Foursome \Four"some\, n.
   1. (Golf) A game between four players, with two on each side
      and each side playing but one ball, the partners striking
      alternately. It is called a mixed foursome when each side
      consists of a man and a woman.
      [Webster 1913 Suppl.]

   2. A group of four people or objects; as, we need another
      player to make a foursome for bridge.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Foursome \Four"some\ (f[=o]r"s[u^]m), a. [Four + 2d -some.]
   Consisting of four; requiring four participants. [Scot. or
   Golf]
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

foursome
    n 1: four people considered as a unit; "he joined a barbershop
         quartet"; "the foursome teed off before 9 a.m." [syn:
         quartet, quartette, foursome]
    2: the cardinal number that is the sum of three and one [syn:
       four, 4, IV, tetrad, quatern, quaternion,
       quaternary, quaternity, quartet, quadruplet,
       foursome, Little Joe]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "foursome":
   Little Joe, biquadrate, bout, contest, decathlon, double-header,
   doubles, event, four, four-part diaphony, game, games of chance,
   go, match, meet, pentathlon, play, play-off, quad, quadrangle,
   quadrature, quadrennium, quadrilateral, quadrille, quadrinomial,
   quadruplet, quartet, quatern, quaternary, quaternion, quaternity,
   quatrefoil, race, rectangle, runoff, singles, sport, square,
   square dance, squaring, tetrad, tetragon, tetragram,
   tetragrammaton, tetrahedron, tetralogy, tetraphony, tetrapody,
   threesome, twosome