The Collaborative International Dictionary of English v.0.48:

4to \4to\ n.
   1. the size of a book whose pages are made by folding a sheet
      of paper twice to form four leaves.

   Syn: quarto
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

4to
    n 1: the size of a book whose pages are made by folding a sheet
         of paper twice to form four leaves [syn: quarto, 4to]