The early lyre of the Greeks had four, constituting their Tetrachord.
"Essays: Scientific, Political, & Speculative, Vol. I"
Herbert Spencer
Caesar, who could have forced him to obey, By his sire's friendship and his own might pray, Yet not draw forth a note: then, if the whim Took him, he'd troll a Bacchanalian hymn, From top to bottom of the Tetrachord, Till the last course was set upon the board.
"The Satires, Epistles, and Art of Poetry"
Horace a.k.a. Quintus Horatius Flaccus Translated by John Conington, M. A.