Conversation so far:
what does the zip in zip code stand for
“ZIP” in ZIP code is an acronym for Zone Improvement Plan.
Does ZIP mean anything else besides Zone Improvement Plan
The USPS intentionally chose the acronym ZIP to indicate that mail travels more quickly when senders mark the postal code on their packages and envelopes

Question: What are the additional 4 numbers for
Self-contained rewrite: What are the additional 4 numbers after the zip code for