Postal Letter

Why does the postal office assign the four digit number after your zip code? What is it for anyway?

Public Comments

1. It identifies your address/mailbox in a numerical format. Theoretically if you mailed something to 55555-4444, it would have no trouble getting to the exact mailbox.
2. It pinpoints the actual structure. Those last four digits are the physical location of your house/apartment on the U.S. Postal Service's streetmaps.
3. The first numbers gets the mail to your city/county and local post office. The last four numbers gets it to your carrier. Helps the post office workers do the job faster and better without getting so stressed they go "postal".
4. I think it specifies the location of your address, more accurately than just the 5 digit postal code. It gives a more precise indication of the area in which the address is located. It probably also makes it easier to sort the mail and get it to the right mail carrier. In some neighborhoods there are several mail carriers, so the codes probably help them to know which mail gets delivered by which mail carrier.
5. It gives additional and more specific destination information.