One word is quartet.
Some others are tetrad and foursome.
If they are four people, a jocular term is "gang of four".
tesseradecades
a series
A big group of books is called a library.I call a group of books a group of books.
A group of books is typically called a library or a collection of books.
A libraryA book group
A library.
A series.
dick
a trilogy
A quartet.
Quartet.
The names of the first four books of the New Testament are Matthew, Mark, Luke and John.
A quartet
A group of four musicians or singers are called a quartet. A group of five musicians or singers are called a quintet.