JSFiddle - React, Tailwind, and code Playground

JavaScript

function largestOfFour(arr) {
    var result = [];

    for (var i = 0; i < arr.length; i++) {
        var max = 0;
        var inner = arr[i];
        for (var j = 0; j < inner.length; j++) {
            if (inner[j] > max) max = inner[j];
        }
        result.push(max);
    }

    return result;
}

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

console.log(output);
alert(output);