function largestOfFour(arr) {
  var largest = [];
  var all = [];
  for(var i = 0; i < arr.length; i++) {
    largest = Math.max.apply(Math, arr[i]);
    all.push(largest);
  }
  return all;
}

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


// SECOND VERSION

function largestOfFour(arr) {
  var finalArr = [];
  for (var i = 0; i < arr.length; i++) {
    var max = Math.max(...arr[i]);
    finalArr.push(max);
  }

  return finalArr;
}

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


// THIRD VERSION

function largestOfFour(arr) {
  var all = [];
  for(var i = 0; i < arr.length; i++) {
    var largest = 0;
    for(var y = 0; y < arr[i].length; y++) {
      if (arr[i][y] > largest) {
        largest = arr[i][y];
      }
    }
    all[i] = largest;
  }
  return all;
}