/ Thesaurus / 4 Synonyms of 4to Find synonyms for: Noun1. quarto, 4to, sizeusage: the size of a book whose pages are made by folding a sheet of paper twice to form four leavesWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: denizen Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language