What’s with Zip (Code) Plus Four? When the Postal Service rolled out this baby, I thought the extra four digits – the tail, if you will – was a “voluntary now, compulsory in a few years” thing. When I’m filling out Internet mail-order forms, the program often adds the tail automatically. But if you asked me to recite the tail for my house, I wouldn’t be able to do it.
Does using the tail get your mail to Point B any faster? Does the tail make mail easier and faster to sort? Do you use the tail?
And what’s with the plus4 often being the same as a PO Box number? If the plus4 indicates a specific area within a zip, as I understood it, how does that relate to a PO Box?
I’m as confused as you are. 
The ZIP+4 thing was a Post Office effort to get material as zeroed-in on the delivery point as possible, for ease of automated sorting. 13202-1011 is one block on East Onondaga Street in downtown Syracuse NY.
Larger corporations often use the +4 tag in their mailings, especially utility company bills, which they of course want to get to the recipient as fast as possible.
It’s convenient to set aside the block of ZIP+4 codings that correspond to that post office’s box numbers to zero in the contents to the appropriate box – they’ll arrive at the local post office sorted out from the stuff that is going out on carrier routes, and can be quickly sorted into the right box.
I use the +4 tail regularly (for us, out in the country, our particular ZIP+4 covers about a quarter mile of the road we live on – us, our landlady, her sister and brother-in-law, the folks in the tenant house, Rick’s home and landscaping company next door, the place he rents out, the neighbors across the street and their daughter and son-in-law’s mobile home next door to them).
Here is the USPS answer:
“Q. Why are ZIP+4 Codes used?
A. 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 misdelivery. 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.”
I use it. If I don’t, my mail gets delivered to the wrong side of town. There are two 2nd Avenues where I live, due to a small boro being incorporated into a larger municipality.
My company is a big mailer. Not utility-company big, but pretty big. ZIP+4 is the best thing evar! When you get to be a big mailer, you pre-sort your own mail. The stuff comes off the printer in ZIP+4 order and we put the stuff straight into the USPS sorting trays. Those trays go directly to the appropriate post office as a unit with minimal handling and the sorters at that level pull out the stacks in ZIP+4 order. Our lost/returned mail rate went way, WAY down when the program picked up steam. Our average time-to-delivery has also gone way down (we deliver statements, not bills, so that’s a customer service thing as opposed to a money collection thing). I think we also get a postage break depending on the volume of presorts.
My father always uses zip+4 for everything he sends out, and also makes typed labels with the postal bar code on them (some word processors can add this). It does indeed result in faster delivery: Mail he sends almost always arrives on the next day. Personally, though, I never use it, since I’m seldom mailing things for which speed is particularly important.