A common misconception is that A4 is the same as 8.5″ x 11″ or Letter Size Paper.
Though they are similar they are very different.
A4 is a standard European size which is 8.26″ x 11.69″
8.5″x11″ or Letter is a standard North American size which is 8.5″ x 11″.
Though there is less than 1″ difference, this formatting will cause some small problems when trying to print on the other size.
Larger problems will arise if your document needs to fit into a sign or document holder and you are unaware of the size.