F4 is a paper format of size 210 mm × 330 mm (8.27 in × 12.99 in). Although metric, based on the A4 paper size, and named to suggest that it is part of the official ISO 216 paper sizes, it appears to be only a de facto standard.
It may be referred to as "foolscap" or "folio" because of its similarity to the traditional Foolscap folio size of 8 1⁄2 by 13 1⁄2 inches (215.9 mm × 342.9 mm).
References
F4 (paper) Wikipedia(Text) CC BY-SA