var a = [11, 2, 3, 4],
    b = [2,  6, 3, 5],
    c = [4, 2, 20, 3],
    d = [34, 2, 21, 5],
    result = [],
    common = [];

function findCommon () {
    for(var i = 0; i < arguments.length; i++){
        var garr = arguments[i];
        result = result.concat(arguments[i]);
    };
}
findCommon(a,b,c,d);

var sorted_arr = result.sort();

for(var i = 0; i< result.length-1; i++){

    if(result[i+1] == sorted_arr[i]){
        common.push(result[i]);
    }
};

alert(common); //[2, 2, 2, 3, 3, 4, 5]

var common=null;

for(i=0; i<a.length; i++)
{
   if(b.indexOf(i) != -1 && c.indexOf(i) != -1 && d.indexOf(i) != -1)
   {
      common = i;
      break;
   }
}