Yeah the last four digits are really not necessary, but if they are there, I do try to include them.
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. Sometimes the extra info does help to expedite the delivery, so put it on there anyway when you can.
AXE