4to vs quarto

4to and quarto both are nouns.

Word	Noun	Adjective	Verb	Adverb
4to	Yes	No	No	No
quarto	Yes	No	No	No

As nouns, 4to and quarto are synonyms defined as:

4to and quarto: the size of a book whose pages are made by folding a sheet of paper twice to form four leaves

4to (noun)	quarto (noun)
the size of a book whose pages are made by folding a sheet of paper twice to form four leaves	the size of a book whose pages are made by folding a sheet of paper twice to form four leaves