Comment by mdaniel

That's a bit chicken and egg; I have my ZIP+4 in 1Password (mostly so I don't forget what it is) but that also means it autocompletes the full ZIP+4 into form fields. I'd wager it's easily 90% of the forms that flag the field as "invalid," forcing me to delete the dash and 4 numbers. And I'm not talking about the setups where there is a separate textbox for the +4, I mean the inputs are always "but a zipcode is 5 digits hurr"

So, with users being actively taught not to provide it, of course no (reasonable? :-D) person is going to know it and thus provide it to make it available for use