diatessaron (n.)

late 14c. as a term in music meaning "interval of a fourth;" 1803 in reference to harmonizings of the four gospels, especially that of Tatian (late 2c.), via Old French and Latin from Greek dia tessaron, from dia "composed of" (literally "through;" see dia-) + tessaron "four," related to tessares "the numeral four" (from PIE root *kwetwer- "four").