Reading a session variable in CI3 that was set by CI4

Hello-  I am working on a large migration from CI3 to CI4.  I have attempted to set the session $savePath and $cookieName to be the same in CI3 and CI4, so that I can read in CI3 a session variable that is set by CI4.  My example is that I have a CI4 page which requires login.  It sets a session variable 'destination' with the URL to redirect to after logging in, then redirects (using a route) to the CI3 login page.  However, once there I am not able to retrieve that session variable.  I'm not sure what I am missing.
I changed app\Config\Session.php to have: 

Code:

    public string $cookieName = 'ci_session';
    public string $savePath = 'ci_sessions_CI3'; 

I confirmed that in CI3 application\config\config.php says:

Code:

$config['sess_driver'] = 'database';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = 'ci_sessions_CI3';
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE;

$config['cookie_prefix'] = '';
$config['cookie_domain'] = '';
$config['cookie_path'] = '/';
$config['cookie_secure'] = FALSE;
$config['cookie_httponly'] = FALSE;

My code in CI4 (app\Controllers\Manage\Admin.php)

Code:

    public function initController(RequestInterface $request, ResponseInterface $response, LoggerInterface $logger) {
        parent::initController($request, $response, $logger);
       
        $this->session = session();         
        $this->db = db_connect();
               
        //Check if this is being run via command line
        if (!$this->request->isCLI()){
            //a user must be logged in to go here so check if they are logged in and are an admin
            if(!$this->ionAuth->loggedIn()){   
                $this->session->set('message', 'Please log in to manage projects');
                $this->session->set('url', current_url()); 
                $this->session->set('destination', current_url());
                // use header because CI4 redirect needs to return and can't return in initController
                header("Location: " . site_url('auth/login'));
                exit;               
            }
            if (!$this->ionAuth->isAdmin()){
                show_404();
            }
        }       
    }

There is a route from auth/login that redirects to CI3 (application\controllers\Users.php). My CI3 code there looks like:  

Code:

        if($this->session->userdata('destination')!=""){
            $this->session->set_flashdata('destination',$this->session->userdata('destination'));
        }
        else{//If not, then just send them to profile management splash page on successful login
            $this->session->set_flashdata('destination',site_url('manage'));
        }
        redirect($this->session->flashdata('destination'), 'refresh');

The above code keeps redirecting me to site_url('manage').  
What am I missing to set the session var in CI4 and read it in CI3?  
Any help is appreciated, let me know if you have more questions.  Thank you!