Refers both to the ensemble of four string players (two violins, viola, cello) and to a piece of music written for that ensemble.
He formed a string quartet.
I like listening to Mozart's string quartets.
A string trio, with three string instruments instead of four.
A string quintet, typically adding a second viola or cello to the quartet.
string quartet
Compound of 弦楽 (string music) + 四重奏 (quartet, literally 'four-fold performance').