Okay, I see that you are using a series of multiple-options questions in a group, each with a single checkbox to select a candidate.
Why don't you put all candidates into one multiple-options question and use the "Maximum answers" setting?
If you need to have separate questions, the following script will limit the number of checked boxes allowed on a page.
Set up your survey to use JavaScript and place the following script in the source of one the questions. Modify the maxChecked variable (line 5) and the alert (line 6) as necessary.
<script type="text/javascript" charset="utf-8">
$(document).ready(function() {
var maxChecked = 4;
var msg = 'You can only select '+maxChecked+' choices';
$('input.checkbox').click(function(){
if($('input.checkbox:checked').length > maxChecked) {
alert (msg);
$(this).attr('checked', false);
}
});
});
</script>
