// The client event AjaxSubmitting takes two parameters sender and e
        // sender  is the object which is raising the event
        // e is the TabItemCancelEventArgs
        function WebTab1_AjaxSubmitting(sender, e) {
            var tab = sender;
            //Gets the index of the tab raising the client event
            var tabIndex = e.get_tabIndex();
            //Gets the collection of tabs
            var tabs = tab.get_tabs();
            //Gets the text of the tab raising the event
            var tabText = tabs[tabIndex].get_text();
            //Assuming you have a label called labelOutput on the form
            var label = $get('labelOutput');
            if (tabIndex == 4) {
                
                //Cancels the client event
                e.set_cancel(true);
                label.innerHTML = "Ajax request cancelled!";
            }
            else
                label.innerHTML = "AjaxSubmitting for tab " + tabText + "...";
        }