Ion-auth 4 routes not working

I used the manual installation method of CI4, and did the same for the updated version of Ion-Auth designed to work with CI4. After copying over the files into my CI4 installation I discovered that it is not routing correctly. When navigating to /auth/login, I get the error message "Controller or its method is not found: App\Controllers\Auth::login". 

I suspect I've not done something correctly related to namespacing, since if I change:

namespace IonAuth\Controllers;
class Auth extends \CodeIgniter\Controller

To

namespace App\Controllers;
use CodeIgniter\Controller;
class Auth extends Controller

It works routes correctly, but I then get the error: "Class 'IonAuth\Libraries\IonAuth' not found".

What am I doing wrong here?