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<div class="unsorted">
    <div rel="5">
      Div 5 here
    </div>
    <div rel="3">
       Div 3 Here
    </div>
     <div rel="4.5">
       Div 4.5 here
    </div>
    <div rel="4">
      Div 4 Here
     </div> </div>
<div class="sorted"></div>
This script will give you the desired output:

$(document).ready(function () {
    var sortedDivs = $(".unsorted").find("div").toArray().sort(sorter);
    $.each(sortedDivs, function (index, value) {
        $(".sorted").append(value);
    });
});

function sorter(a, b) {
    return a.getAttribute('rel') - b.getAttribute('rel');
};