Can session autoload?

The BaseController is intended to be the mechanism for providing the equivalent of autoloading from CI3 ... https://codeigniter4.github.io/userguide...oller.html

Your first post above is trying to create a Sessions namespace, which is not provided for in the framework.

CI4's app/Config/Autoload is meant to tell the PHP class loader where to find source code, and not to preload components for a request.