Ah, but it is. It's used everyday for residential mail, check. There is most likely a bar code sprayed on the face of the mail piece and it contains your zip+4 plus other information. Depending on the type of mail, there may be carrier route codes. There's a lot of information in that one line of bar code. :)
It's been like this for many years and it still hasn't caught on. So, the Post Office takes care of it from their end. All mail is scanned, regurgitated and sprayed. It is then delivered from there.
I know my zip+4. I know all my clients zip+4. I probably visit the zip+4 interface at least once a week to verify an individual address.
All mail that is mailed in bulk is required to follow certain guidelines. Zip+4 is one of those. Personal mail is exempt at this time but the Post Office is going to spray it prior to arriving in your mailbox.
Just search Google for zip+4 (or bookmark the site). Enter your address, city, state, it will return the proper zip+4 for your address. You will need to enter Suite numbers in most instances to further refine the zip+4 and the interface will let you know that.
I've been writing my zip+4 for many years. And then again, I write my dates like 2008-01-10. ;)