Quartet.
Four people singing is often called a quartet.
A group of four people singing is called a quartet.
== Trio== ] ==
A quartet
Quartet.
Four people singing is often called a quartet.
A group of four people singing is called a quartet.
== Trio== ] ==
A quartet
Trio
Choir
A sextet.
A duet.
two people singing together is a duet
The noun for a four person singing group or musicians is a quartet.
The general term would be a quintet.