Listen to World of Envelopes - A4 Envelope Size – A Guide For Marketers

The Most Common Envelopes designed to hold A4 paper are:
C4 envelope– designed to fit an A4 paper document, it encloses a full sheet without the need of folding it.
C5 envelope- designed to hold an A4 piece of paper folded in half or unfolded A5 paper.

If you’re looking for the A4 envelope size just visit: http://www.worldofenvelopes.com/a4-envelope-size.html