quartet

noun4

1. •
   A group of four people or things.

   • The last of the quartet of American fighter pilots to join the RAF before the war was Cyril 'Pussy' Palmer, who was born in Cleveland, Ohio, in March 1918. — 2015
2. •
   A group of four musicians who perform classical music together.

   • The string quartet played some chamber music.
3. •
   A group of four singers that sing together in four-part harmony.

   • The barbershop quartet performed the composition a cappella.
4. •
   A musical composition in four parts, each performed by a single voice or instrument.

   • He composed a string quartet.