A series of four books is called a tetralogy.
The word ‘trilogy’ means ‘a series of three works’. It comes from the Greek trilogia, which is made from two word-forming elements: tri- meaning ‘three’ and logia, which is from logos, meaning ‘story’, ‘idea’, ‘speech’.
In order to find a corresponding word that means ‘a series of four works’, we just replace tri- with the Greek numeral prefix meaning ‘four’, which is tetra-. Hence, a tetralogy is a series of four works (or books).
Logos was also a word in Latin (Latin borrowed many words from Greek), so you can alternatively use the Latin numeral prefix for ‘four’, which is quadri-. So a series of four works can also be called a quadrilogy.
A series of four works is also sometimes called a ‘quartet’.
Other words for series’ of works
|A series of two works||duology|
|A series of three works||trilogy|
|A series of four works||tetralogy|
|A series of five works||pentalogy|
|A series of six works||hexalogy|
|A series of seven works||heptalogy|
|A series of eight works||octology|
|A series of nine works||ennealogy|
|A series of ten works||decalogy|
|A series of eleven works||hendecalogy|
|A series of twelve works||dodecalogy|