More than likely the admin directory within the EP4 package has been copied to YOUR_SECRET_ADMIN folder rather than the contents of the admin directory within the EP4 package being copied there.

If you locate the file easypopulate_4.php on your server, what other file(s) are in the directory with it? If they are only other EP4 files, then loaded to the wrong location.

Installation of EP4 with regards to loading the contents of the admin directory to your server's admin directory is very similar to the installation of any other ZC plugin. It is an admin side application with no catalog side facing software (no basic includes folder).