protected void Application_Error(object sender, EventArgs e)
{
    HttpException httpException = Server.GetLastError() as HttpException;
    if (httpException.ErrorCode == 404)
       Response.Redirect("/MainPage.aspx");
}

<customErrors mode="RemoteOnly" defaultRedirect="Error.aspx">
    <error statusCode="404" redirect="MainPage.aspx" />
</customErrors>