[SOLVED] CI4 - Unable to Autoload Markdown Class

Did you install markdown using Composer? Is so, and if I understand correctly, then you do not need to add anything to /app/Config/Autoload.php. Doing so may be the source of your problem.

CI4 is hardwired to use the composer autoloader which will work just fine in conjunction with the C4 autoloader.

Using require_once is only appropriate when not using an autoloader.