A Dictionary of Music and Musicians/Fourth
FOURTH is an interval comprising two whole tones and a semitone. It is called a fourth because four notes are passed through in going from one extreme of the interval to the other, for which reason the Greeks called it δια τεσσαρων—Diatessaron. The ratio of the vibrational numbers of its limiting sounds is 3:4. It is in fact a perfect consonance, though regarded as a discord in the old Diatonic style.
[ C. H. H. P. ]