<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
div{
padding: 1%;
background-color: #CD853F;
}
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function() {
var a = 4;
var f=$("#x" + a);
$('<input/>').appendTo(f)
});
</script>
</head>
<body>
<div id="x4"></div>
</body>
</html>