noun
Refers to the group of four musicians playing string instruments, not the musical composition.
彼は弦楽四重奏団のチェロ奏者です。
He is the cellist of a string quartet.
弦楽四重奏 refers to the musical composition or the genre, while 弦楽四重奏団 specifically refers to the ensemble of performers.
Compound of 弦楽 (string music), 四重奏 (quartet), and 団 (group).