function largestOfFour(arr) {
  // You can do this!
  var resultArr = [0];
  var largest = [0];
  var index = 0;
  for (var i = 0; i < 4; i++){
    for (var j = 0; j < 4; j++){
      index = 0;
      if (arr[i][j] > index){
        index = arr[i][j];
      }
      resultArr[i] = index;
    }
  }
  console.log(largest);
  return resultArr;
}

largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);