A4 is a standard paper size, defined by the international standard ISO 216. It is the normal size of paper for both domestic and business purposes in all countries except the United States and Canada. See paper size for other paper sizes.
It is 210 x 297 mm.
It is sometimes claimed that the aspect ratio of this paper format corresponds to the golden mean (1.618, approximately). This is an error; in fact the aspect ratio for all international standard paper sizes is defined as the square root of two, which ensures that if the paper is folded in half, the halves are the next standard size down:
The A4 road in England runs from London to Bristol.
A4 is also the actual model number for the German V-2 rocket.