[Sweet]

Можно сделать гораздо "дешевле", за один проход:

<div class="N" id="3"> </div>
<div class="N" id="1"> </div>
<div class="N" id="4"> </div>
<div class="N" id="2"> </div>

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.js"></script>
<script>

var $elements = $(".N");
var element = Array.prototype.reduce.call($elements, function (revious, current) {
    return current.id > revious.id ? current : revious;
});

alert( element.id );

</script>