Probably something like this

$(document).ready(function() {
    $("#q4 input").prop("disabled", true);
    $(".Submit").prop("disabled", true);
    
    $("#q5 a").click(function() {
        $("#q4 input").prop("disabled", false);
        $(".Submit").prop("disabled", false);
    });
});

"a" is the element for the link and #q5 is the id of the parent "li" element.  ".Submit" is the class for the submit button.  "input" is the element for the checkbox and "#q4" is the id of the parent "li" element.

The form would disable the submit button and checkbox on completion of document load, and then the "click" action handler for the link re-enables them.

Check out https://api.jquery.com/