[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)