A ZIP + 4 code uses the basic five-digit code plus an additional four digits to identify a geographic area inside the five-digit delivery area , such as a group of apartments , or a location which gets a large amount of mail . 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 .