The four numbers at the end of a zip code are used for delivery sectors and segments. These sectors or segments may be city blocks, a group of streets, buildings or apartments, or any structure that can be used as a landmark for delivery.
The four numbers at the end of the zip code were first introduced in 1983 and named "ZIP+4." These four extra numbers were required for mailing packages and letters when the idea was introduced, but after some time, people had not accepted the idea and it became optional. USPS offers a webpage at USPS.com to look up the zip code for any address, including the last four digits.