function flatten(ary) {
  var ret = [];
  for(var i = 0; i < ary.length; i++) {
    if(Array.isArray(ary[i])) {
      ret = ret.concat(flatten(ary[i]));
    } else {
      ret.push(ary[i]);
    }
  }
  return ret;
}

var arrays = [[1,2,[3]],4];

console.log(flatten(arrays));
// [1, 2, 3, 4]