From WordNet (r) 2.0 (August 2003) [wn]:
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]
2: the cardinal number that is the sum of three and one [syn:
four, 4, IV, tetrad, quatern, quaternion,
quaternary, quaternity, quartet, quadruplet,
Little Joe]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]