一个关于ModalPopupExtender的问题

为什么点击Button4的时候没有反应？
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default4.aspx.cs" Inherits="Default4" %>
<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="cc1" %>
<!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 runat="server">
    <title>无标题页</title>
    <script type="text/javascript">
        function tt() { 
        alert("ff");
    }
    function dd() {
        alert("dd");
    }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Button ID="Button3" runat="server" Text="Button" />
    <asp:Button ID="Button4" runat="server" Text="Button" />
     <asp:Panel ID="panel1" runat="server">
       <div>
        fsagsrggggg
           <input id="Button1" type="button" value="button" />
           <input id="Button2" type="button" value="button" />
       </div>
     </asp:Panel>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <cc1:ModalPopupExtender ID="Panel1_ModalPopupExtender" runat="server" DynamicServicePath=""
        BehaviorID="panel1"  PopupControlID="panel1" Enabled="True" TargetControlID="Button3" OkControlID="Button1" CancelControlID="Button2" OnOkScript="tt()">
    </cc1:ModalPopupExtender>
     <cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" DynamicServicePath=""
        BehaviorID="panel1"  PopupControlID="panel1" Enabled="True" TargetControlID="Button4" OkControlID="Button1" CancelControlID="Button2" OnOkScript="dd()">
    </cc1:ModalPopupExtender>
    </div>
    </form>
</body>
</html>