A4 n (Printing, Lithography & Bookbinding) a. a standard paper size, 210 × 297 mmb. (as adjective): an A4 book. See also A sizes Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014