(ZIP+4 or ZIP plus 4) Extension of the United States postal code providing four extra digits for additional location specificity.

A ZIP+4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery.

ZIP+4 on Wikipedia.

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