ho capito finalmente, anche se non mi è chiaro il perchè nel valore non posso metterci solo href....

sicuramente ci sarà un motivo ma siccome javascript non riesco mai a capirlo al 100%, va bene così

grazie Andrea, mi hai dato una grande mano

codice:

<!DOCTYPE html>
<html>
    <head>
    
    </head>
    
<body>
    
    
    
<a class="obclass3" href="http://www.link3.html"></a>
<a class="obclass2" href="http://www.link2.html"></a>
<a class="obclass1" href="http://www.link1.html"></a>
<a id="object" class="obclass0"  href="http://www.link0.html"></a>




    <button onclick="toggle()"><h3>clicca</h3></button>
    
    
<style>
    .obclass0{
    width:300px; 
    height:300px;
    background-image: url();
    background-color: rgb(0, 0, 0);
    background-repeat: no-repeat; 
    background-attachment: scroll;
    position:absolute;
     }
    .obclass1{
    width:300px; 
    height:300px;
    background-image: url();
    background-color: rgb(141, 14, 14);
    background-repeat: no-repeat; 
    background-attachment: scroll;
    position:absolute;
     }    
    .obclass2{
    width:300px; 
    height:300px;
    background-image: url();
    background-color: rgb(43, 141, 14);
    background-repeat: no-repeat; 
    background-attachment: scroll;
    position:absolute;
     }     
      .obclass3{
    width:300px; 
    height:300px;
    background-image: url();
    background-color: rgb(42, 62, 150);
    background-repeat: no-repeat; 
    background-attachment: scroll;
    position:absolute;
     }     
   
  
    
button{
    top:310px;
    position:absolute;
     }
    
    
    </style>




<script>
function toggle() {
       var object=document.getElementById("object");
       if (object.className=="obclass0"){
           object.className="obclass1",
               object.href="http://www.link1.html"}
     else  if (object.className=="obclass1"){
         object.className="obclass2",
             object.href="http://www.link2.html"}
     else  if (object.className=="obclass2"){
         object.className="obclass3",
             object.href="http://www.link3.html"}
     else if (object.className=="obclass3"){
         object.className="obclass0",
             object.href="http://www.link0.html"}
      


}
</script>


</body>
</html>