Biraz uğraştım bonus kartın kodlarından alabildim.

 var focused_input="";
   var focus_name="";
   var f = document.forms['giris'];
   var form_element;
   var currentField;
   var len=0;

   function focusNext() {
      for (form_element=0; form_element <= document.forms['giris'].elements.length; form_element++) {
         if (document.forms['giris'].elements[form_element].name == focused_input) {
            form_element++;
            break;
         }
      }

      while (document.forms['giris'].elements[form_element].disabled)
         form_element++;
      focus_name = "document.forms['giris'].elements[form_element].focus()";
   }

   // Bunlar klavyeden girişler için
   function advance(c) { 
      currentField = c;
      // Şifre ve CVC2 alanlarının keypadden girilmesi mecburi
      if (currentField.name.indexOf("SIFRE")!=-1 || currentField.name.indexOf("EKKOD")!=-1) {
         eval('document.giris["' + currentField.name + '"].value="";');
         eval('document.giris["' + currentField.name + '"].focus();');
         alert("Bu alan bilgisini Mini Klavye'yi kullanarak girmeniz gerekmektedir.");
         return;
      }
      eval('document.giris["' + currentField.name + '"].focus();');
      setTimeout('effect()',1);
   }

   function effect() {
      if (currentField.value.length == len) {
         focusNext();
         eval(focus_name);
      }                                    
      else 
         currentField.focus();
   }

<td align="left" bgcolor="#EEEEEE"> 

        <input name="textKARTNO1" type="text" class="blacktext" size="4" maxlength="4" onFocus="focused_input='textKARTNO1';focusNext();" onKeyUp="len=4;advance(this);">

        <input name="textKARTNO2" type="text" class="blacktext" size="4" maxlength="4" onFocus="focused_input='textKARTNO2';focusNext();" onKeyUp="len=4;advance(this);">

        <input name="textKARTNO3" type="text" class="blacktext" size="4" maxlength="4" onFocus="focused_input='textKARTNO3';focusNext();" onKeyUp="len=4;advance(this);">

        <input name="textKARTNO4" type="text" class="blacktext" size="4" maxlength="4" onFocus="focused_input='textKARTNO4';focusNext();" onKeyUp="len=4;advance(this);">

      </td>

Sizde burdaki uygulamadan yararlanarak yapabilirsiniz