Excel 2007 : Zipcode sorting problem

My database has mix of 5 digit zip code and zip+4. Also, after export from address book lost leading "0"s.
I inserted a column next to the zip code column, formatted it for "text", copy/pasted the zip code field and added the leading "0" manually.
When I sort, the zip codes that have the leading "0" sort correctly merging the 5 and 9 digit zips appropriately BUT after the "9"s.
All the others (starting with 2-9) sort 5 digit first then 9 digit.
Would appreciate help as this is being prepared for a bulk mailing. Thank you!