quarto (n.)

"book from paper folded twice to make four pages to the sheet, size of book in which the leaf is one-fourth of a certain size of paper," late 15c., in the phrase in quarto, from Medieval Latin in quarto "in the fourth (part of a sheet of paper)," from quarto, ablative singular of Latin quartus "the fourth, fourth part" (related to quattuor "four," from PIE root *kwetwer- "four").