Copy the value of one input box to another input box only if checkbox is checked

indow.onload = function() {
    var src = document.getElementById("one"),
        dst = document.getElementById("two");
    src.addEventListener('input', function() {
        dst.value = src.value;
    });
};

$(function () {
    var $src = $('#three'),
        $dst = $('#four');
    $src.on('input', function () {
        $dst.val($src.val());
    });
});