JSFiddle - React, Tailwind, and code Playground

JavaScript

function largestOfFour(largestOfFour) {
    largestOfFour.forEach(function(number, index){
        number.sort(function(a, b){
            return b - a;
        });
        largestOfFour[index] = number[0];
    });
    
    alert(largestOfFour);
};

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