获取或设置GridView中的Label值

[feishanm]

<asp:GridView ID="GridView1" runat="server" AllowPaging="True" AutoGenerateColumns="False" DataSourceID="SqlDataSource1" PageSize="20">
<Columns>
<asp:TemplateField HeaderText="开始日期" SortExpression="ksrq">
<ItemTemplate>
<asp:Label ID="Label4" runat="server" Text='<%# Bind("ksrq","yyyy-MM-dd") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>

请问我在后台代码中如何得到Label4的值或改变它的值。
好像不能直接用Label4.Text = "XXX"
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
//Label lb = (Label)e.Row.FindControl("Label4");
Label lb = (Label)GridView1.FooterRow.FindControl("Label4");
lb.Text = "aaa";
}这要写总是不行，提示错误：“未将对象引用设置到对象的实例。”

[amandag]

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.Footer)
{
Label lb = (Label)GridView1.FooterRow.FindControl("Label4");
lb.Text = "aaa";
}
}

[feishanm]

多谢，结帖