It literally means four (tetra) graphs (gramma). The expression refers to the consonantal name of Yaweh (YHWH), which was expressed symbolically as an expression of his identity. In Hebrew there are no vowels, thus leaving phonetics linked to oral tradition. From the third century the Masoretic school added periods and apostrophes to indicate the correct pronunciation.
Pedro Chico González, Dictionary of Catechesis and Religious Pedagogy, Editorial Bruño, Lima, Peru 2006
Source: Dictionary of Catechesis and Religious Pedagogy