Javascript-Round number to decimal

Hi,

I would like to round a calculated value (var Sum) to 4 decimal spots. The JavaScript formula I am using to get the value is below:

<script type="text/javascript">

 function calc() 
{ 
 var Sum = 0; 
 Sum = Number(document.getElementById("Q3_r1").value) 
 * Number(document.getElementById("Q3_r2").value/ [% Q1 * Q2 * 52 %])  
 document.getElementById("Q3_Total").value = Sum;
 } 
 </script>