codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
<script type="text/javascript"> 
<!-- 
function conta(){
var sel=0;
    var box = document.getElementsByTagName('input');
    for(i=0; i<box.length; i++){
        if(box[i].getAttribute("type") == 'checkbox' && box[i].checked){
            sel++}
    }
    for(a=0; a<box.length; a++){
            if(sel>4 && !box[a].checked){
                box[a].setAttribute("disabled","disabled");
            }else{
                box[a].removeAttribute("disabled");
            }
    }
}
 //--> 
</script> 
</head>

<body>
<div id="test">
    <input name="ck1" type="checkbox" onclick="conta()" value="" />
<input name="ck2" type="checkbox" value="" onclick="conta()" />
<input name="ck3" type="checkbox" value="" onclick="conta()" />
<input name="ck4" type="checkbox" value="" onclick="conta()" />
<input name="ck5" type="checkbox" value="" onclick="conta()" />
<input name="ck6" type="checkbox" value="" onclick="conta()" />
<input name="ck7" type="checkbox" value="" onclick="conta()" />
<input name="ck8" type="checkbox" value="" onclick="conta()" />
<input name="ck9" type="checkbox" value="" onclick="conta()" />
<input name="ck10" type="checkbox" value="" onclick="conta()" />
</div>
</body>
</html>