From BiblePortal Wikipedia

Webster's Dictionary [1]

(n.) The mystic number four, which was often symbolized to represent the Deity, whose name was expressed by four letters among some ancient nations; as, the Hebrew JeHoVaH, Greek qeo`s, Latin deus, etc.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]

( Τέτταρα , Four, and Γράμμα , Letter), a term to designate the sacred name of the Deity, Jehovah, in four letters, יהוה . By the possession of this name the early Jewish opponents of Christianity declared that the miracles of Christ were performed. Tile mystical word Om of the Buddhists of India and Thibet is supposed to possess similar virtues to the present day.

The Nuttall Encyclopedia [3]

The mystic number "four," symbolical of deity, whose name in different languages is composed of four letters.