[bluGill]

They are public, but the post office changes the last 4 digits every few months so there is no point in telling anyone what yours is. These days the post office can look up your street address and give you all the information they need - which is an 11 digit bar code good for the next week.

[aaronax]

Based on my personal experience, I really doubt that the last 4 digits of the ZIP+4 are changing more often than once per decade or longer. I could see the delivery point of the 11-digit code changing every few months, but you are already aware of that code system so it is not simple confusion between the two on your part.

Could you provide more information or a source?

[bluGill]

20 years ago they changed all the time. Wikipedia doesn't mention this though. These days the post office can read the street address via computers and get the 11 digit code they need, so I suspect they don't need them. (for PO boxes the 9 digit code apparently doesn't change)

[terribleperson]

My 9-digit zip hasn't changed in at least 10 years.

[sroussey]

Mine has not changed in 25 years.