the +4 zip is not unique to a mail stop.
it is unique to an area.
the size of the area depends on the location.
your previous information is incorrect.
the information given you by the Post Office is correct.
they have no reason to lie to you.
Please accept my answer.
for more detail, please read the information on this link http://en.wikipedia.org/wiki/ZIP_code
thank you for the accept,