What are the last 4 digits of the ZIP code?

The last 4 digits after a 5-digit ZIP code provide a way for the Post Office to further sort the mail. For a given ZIP+4 code, there may be only a few houses (or apartments). Not always, but they often are in order of the way that the carrier drives his/her route.

While you cannot look at the 4-digits and find them on a map, the Post Office has a database of every address and what carrier route delivers to that address, and where it is in relation to the whole route.