if you want to use
<form runat=server target=below runat="server">
<asp:Button id=btn runat=server text="提交" />
</form>
then ASP.NET will ignore your action setting, since the recommended way is postback to the current page, if the target page is on the same app, then save the data in session variables or some other storage and do Response.Redirect, or do Server.Transfer
or you can try (not recommended):
<form runat=server target=below runat="server" onsubmit="this.action='frame4.aspx';">
and in frame4.aspx, you might need to do
<%@ Page EnableViewStateMac="false" %>