<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>// ver 1
var jQ = $.noConflict(true);
jQ(document).ready(function(){
// ****Uncomment the line below after testing code *** //
//jQ('input[name*="phone_xxx"]').parent().parent('[id^="item"]').css('display','none');
jQ("input[name='phone']").on('change', function() {
var phone = jQ(this).val();
var regex = /\d(?=(?:\D*\d){4})/gm;
var subst = 'x';
var phone_xxx = phone.replace(regex, subst);

jQ("input[name='phone_xxx']").val(phone_xxx);
});
});</script>