Quartet
Part of speech: noun
Definition: A music composition in four parts, each performed by a single voice or instrument. The set of four musicians who perform a piece of music together in four parts. A group of four.
Example sentence: I owe very, very much to Mozart; and if one studies, for instance, the way in which I write for string quartet, then one cannot deny that I have learned this directly from Mozart. And I am proud of it!
Foursome
Part of speech: noun
Definition: A group of four, a quartet or a game (such as golf) played by four players, especially by two teams of two.