models in CI4 - OK to put in app/Models? What about naming/namespacing/autoloading?

You can still create your own models without using CodeIgniter's Model.

EDIT: like many of your other responses, this one is not especially informative. I was rather hoping that someone might read my entire post and comment with something a bit less terse and more in the spirit of discussion.

I would also point out that the documentation I linked in my original post makes it clear that one need not extend CodeIgniter\Model:

You do not need to extend any special class to create a model for your application. All you need is to get an instance of the database connection and you’re good to go. This allows you to bypass the features CodeIgniter’s Model gives you out of the box, and create a fully custom experience.

PHP Code:

<?php namespace App\Models;

use CodeIgniter\Database\ConnectionInterface;

class UserModel
{
    protected $db;

    public function __construct(ConnectionInterface &$db)
    {
        $this->db =& $db;
    }
}