Using update panel with master & nested master pages

[katmagic]

How do I use the UpdatePanel with 2 content sections on the content page,
each section should update the other section. I keep getting the "Control
not found for asynchronous postback trigger" error. In the below example, I
want the gridview in each control section to update the other gridview.

<%@ Page Language="vb" AutoEventWireup="false" EnableEventValidation="false"
MasterPageFile="~/MasterNested.master" CodeBehind="stuff.aspx.vb"
Inherits="Main.Stuff"
title="Stuff" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit"
TagPrefix="cc1" %>
<asp:Content ID="Content1" ContentPlaceHolderID="MasterNestedHead"
runat="server">
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MasterNestedHeadings"
runat="server">

<asp:Label id="StatusMessage" runat="server"></asp:Label><br />
</asp:Content>

<asp:Content ID="Content3" ContentPlaceHolderID="MasterNested"
runat="server">
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
Gridview here for content3
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID=Gridview4 />
</Triggers>
</asp:UpdatePanel>

</asp:Content>

<asp:Content ID="Content4" ContentPlaceHolderID="MasterNestedOrderDetails"
runat="server">

Gridview here for content 4

</asp:Content>