<script>
    (function(){
        var hadr = document.querySelector(".h-adr");
        var onKeyupCanceller = function(event){
            if(!event.target.value.match(/^420[0-9]{4}$/)){
                event.stopImmediatePropagation();
            }
            return false;
        };
        var postalcode = hadr.querySelectorAll(".p-postal-code");
        var postalField = postalcode[postalcode.length - 1];

       postalField.addEventListener("keyup", onKeyupCanceller, false);

    })();
</script>