To determine a ZIP +4 Code you will need to know an address. The last four digits are not specific to a city but to a street address or Post Office Box.
Because of identity theft and concerns, many businesses ask for only the last four digits of our . There seems to be an assumption that only four digits of a nine-digit-number shouldn’t be a concern.
For Post Office boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often either the last four digits of the box number or 0 plus the last three digits of the box number. However, there is no uniform rule, so the ZIP+4 code must be looked up individually for each box.