splitBy.js

function splitBy (by, str) {
    return (function splitByFourAux (input, index, acc, string) {
        return index > input.length -1 ?
            (string ? acc.concat(string) : acc) :
            index % by === by - 1 ?
                splitByFourAux(input, index + 1, acc.concat(string.concat(input.charAt(index))), '') :
                splitByFourAux(input, index + 1, acc, string.concat(input.charAt(index)));
    })(str, 0, [], '');
}

splitBy(3, '222244443333'); //["222", "244", "443", "333"]
splitBy(2, '222244443333'); //["22", "22", "44", "44", "33", "33"]

Although, you could do:

str.split(/(\d{4})/).filter(Boolean) // By groups of 4

XD