[AccessD] Zip+4?

Group

I use a separate tblGeo table to manage 
city/state/post/country/region/timezone/dst/lat/long data with an fk join on 
tblOrgAddress.
This is presented as an enforced combo with any new org address entry, thus 
ensuring that only valid entries are allowed for that data.
When US zip codes were only 5 digits, this worked quite well ...but the 
growing number of +4 zips is beginning to grow out of control ...adding the 
+4 means an additional 10k entries per US post code are possible ...which 
would drive the db to its knees.

So I've separated out the +4 and isolated them in the tblOrgAddress to limit 
the impact on lookups and just concatenate them when I assemble addresses. 
This is of course time and code intensive in its own right but limited to 
only those times when a full mailing address is required.

The problem comes in requiring data entry people to enter the +4 separate 
from the post code ...it just isn't getting done ...and the client is now 
seeing address rejections from the postal service bulk mailing because the 
+4 is missing.

Question is, does anyone have a better way of handling the zip+4 issue?

William