What is a set of 4 books called?

Could you please elaborate on the concept of a set of 4 books? I'm curious to know what term or phrase is commonly used to describe a collection of four books that are often grouped together, whether for a specific theme, series, or other purpose. Is there a specific name for such a grouping in the world of literature or publishing? I'm interested in learning more about this terminology and how it's applied in various contexts.