var num = '11'
var n = num.length;
num = n < 4 ? (n == 1 ? '000'+num : (n == 2 ? '00'+num : '0'+num)) : num;
alert(+num);

var str = new String(sum);
		while(str.length < 4)
			str = '0' + str;

function addZeroes(str) {
  return ('0000' + str).slice(-4);
}
alert(addZeroes('10'));
alert(addZeroes('100'));
alert(addZeroes('1'));