F4 is a paper format of size 216 by 330 mm (8.5 by 13 in). Although metric, based on the A4 paper size, and named to suggest that it is part of the official ISO 216

ISO 216

ISO 216 specifies international standard paper sizes used in most countries in the world today. It defines the "A" and "B" series of paper sizes, including A4, the most commonly available size...

 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

Foolscap folio

Foolscap folio is paper cut to the size of 8 1/2 × 13 1/2 in...

size of 8+1/2 ×.