What is the purpose of the last 4 digits?

From:  "C++gUrU"

What is the use of the last four digits in the United States zip codes?

So here is the list of responses that we received. Basically heres what they all add up to... the extra four numbers after the five digit zip codes purpose is to define a smaller delivery area allowing the item to reach its destination.

TB:  To define a finer delivery area. 5 digit zip codes can be a whole town or small city. Zip plus allows sorters to separate letters / packages into smaller areas making faster delivery possible.

ElbaliavanU:  The last 4 digits after a ZIP code are part of the zip code. ZIP stands for "Zone Improvement Plan", it is put after an address so that the address is easier to find, and the mail gets there faster. the east coast has zip codes starting with 1, and as it goes west the zip codes get up to starting with 8 or 9 (example, 12345 somehwere on the east coast, and 87654 on the west coast). the 4 numbers after it are the zip code area divided into even smaller areas, and each 4 digit code is different in each zip code area. it gets the mail to your house even faster than the regular zip code.

Todd:  I believe you mean last four as in 55555-1111, the last four being the 1s. Those last four are almost ZIP code ZIP codes. Its a way to divide up cities by smaller sections. Just as a ZIP code is a city, the last four are sections of a city. The last four are not required to be used. Although using them may speed up delivery. You can actually visit this website and find the last four digits.

Paul:  In 1983, the Postal Service began using an expanded ZIP Code called "ZIP+4." A ZIP+4 Code consists of the original 5-digit ZIP Code plus a 4-digit add-on code. The 4-digit add-on number identifies a geographic segment within the 5-digit delivery area, such as a city block, office building, individual high-volume receiver of mail, or any other unit that would aid efficient mail sorting and delivery. Use of the 4-digit add-on is not mandatory, but it helps the Postal Service direct mail more efficiently and accurately because it reduces handling and significantly decreases the potential for human error and possibility of mis-delivery. It also will lead to better control over USPS costs and, in turn, postage rate stability. ZIP+4 is intended for use primarily by business mailers who prepare their mail with typewritten, machine-printed, or computerized addressing formats that can be read by the Postal Service's automated scanners during processing. Mailers who qualify receive a rate discount on First-Class, non-presorted, ZIP+4 mailings of at least 250 pieces and on presorted ZIP+4 mailings of at least 500 pieces. There are also ZIP+4 discounts for bulk business mail.

Xach:  The last four digits of an area code are to furter narrow done the area for which the address is in. there really not that important to normal people

duncan:  The purpose of the "+4 code" is to speed up the sorting of the mail. The first to digits I believe are the mail carrier number and the second 2 are the stop.

i know all:  the first five are the area and the last four is the exact house