What a great idea! The closest you can get to this in the system is with a "contains" rule. See attached screen-shot below. However this rule could end up including other zip codes that end in those last three digits. So not exactly ideal However I can see the utility in having a "begins with" option in that second drop down. I've emailed product management about this internally however features are always best requested by customers from:https://forums.adobe.com/community/experience-cloud/marketing-cloud/target/ideas if you let me know in a reply here after you submit the idea I'll make sure to vote for it!
Mihnea Docea | Technical Support Rep, Customer Care | Customer & Employee Experience | Adobe | (:: 1 (800) 497-0335
Target Geo audiences actually use a third party license from DeviceAtlas, so this may not be possible right out of the box. However, you may be able to combine the default audience, with a custom profile script to assist suit your audience needs, to contain only a specific number of of digits.