It seems that aMember likes to truncate zipcodes with the additional 4 digits. It's not a limitation with the members table. I can enter my zip code "48073-4607" and if I browse my record in aMember, the last two digits are missing.
edit amember/templates/admin/user_form.html replace line PHP: <td><input type=text name=zip value="{$u.zip|escape}" size=6 maxlength=8></td> to PHP: <td><input type=text name=zip value="{$u.zip|escape}" size=16 maxlength=16></td>