api for zip +4 from an address

have you tried Google Maps JavaScript API V3

UPDATED:

in responce to your comment

this is easy as count 1, 2 , 3 ;)

take a look at this:

• http://maps.google.com/maps/api/geocode/json?address=Winnetka&sensor=false

you need to looking for google map geocoding service! ( Viewport Biasing )

• http://code.google.com/intl/it-IT/apis/maps/documentation/geocoding/

example code would be:

using jQuery

$(function() {
    $.getJSON("http://maps.google.com/maps/api/geocode/json?address=Winnetka&sensor=false",
    function(data) {
        var zip_code = data.results[0].long_name;
        alert(zip_code);
    });
});

Yahoo has a zip + 4 in thier API, limit 5000 request per day.

Yahoo GeoCoding

The USPS has an API for finding/checking zip codes (among other things).

http://www.usps.com/webtools/address.htm