That is an interesting point. Why is the #4 missing more often than the other numbers? And on so many coins. Does this occur across all the Mints, or is it unique to one? Today, when a new coin is introduced, it is the result of lengthy development. But the one year (1943) alteration, to and from steel, was rushed... perhaps quality of the die or steel was an issue.
Obviously, no one knows the answer to this interesting question, but it does add a bit of mystery, as well as history, to the coin. Although it's not considered an error, it is a distinctive coin to include in a collection.
