Nearly everything in the US uses letter, legal (letter but longer), or tabloid (double width letter, to be folded over).
Much to my surprise, a random check of a US-based office supply company shows that they do have A4 in stock -- at a price about 40% higher than letter-sized.
Much to my surprise, a random check of a US-based office supply company shows that they do have A4 in stock -- at a price about 40% higher than letter-sized.