<SCRIPT LANGUAGE="JavaScript">
function documentClick()
{
var e = window.event.srcElement;
if (e.tagName=="INPUT" && e.type=="checkbox" && e.checked)
{
var a = document.getElementsByName(e.name);
var n = 0;
for (var i=a.length-1; i>-1; i--)
{
if(a[i].checked) n++
}
if (n==4) e.checked = false;
}
}
document.attachEvent("onclick", documentClick);
</SCRIPT>