Need fix the precision for a value through JavaScript

I have a variable with one number like below.
<script>
var v = "33.333333333333"
</script>
Through JavaScript i have to make the precision to 4.
i mean i have to make var v="33.3333"
Could Some body give information.....
Thanks in Advance

look at this post for rounding money...
http://www.coderanch.com/t/113985/HTML-JavaScript/Rounding-off-amounts