Quarto (sometimes abbreviated to "4to"), a term from bibliography, refers to the format of a book. A quarto is the second largest format, coming in between the folio and the octavo. A sheet is folded over twice, so each sheet consists of four leaves (or eight pages). A typical quarto is a bit larger than the average hardcover book today.

Just as the quarto falls between the folio and the octavo in size, it falls between them in dignity. The most serious materials (such as Bibles) have traditionally appeared in folios, whereas more popular works (such as novels) appeared in octavos.

From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.