function unite(arr1, arr2, arr3,arr4) {
  
  // reduces & combines all arrays
  var uniteall = arr1.concat(arr2).concat(arr3).concat(arr4);
  
  // snippet to remove duplicated values
  function uniq_fast(a) {
    var seen = {};
    var out = [];
    var len = a.length;
    var j = 0;
    for(var i = 0; i < len; i++) {
         var item = a[i];
         if(seen[item] !== 1) {
               seen[item] = 1;
               out[j++] = item;
         }
    }
    return out;
}

  // running values into the function
  var uniteall2 = uniq_fast(uniteall);
  
  // clearing undefined values
  var cleararr = uniteall2.filter(function(n){ return n !== undefined; });
  
  // output
  return cleararr;
}

// test
unite([1, 2, 3], [5, 2, 1]) ;