Consider the following project structure:
When I write
$axel->addModule(new Axel\Module\PSR4('../Model', '\\Model'));
in my index.php, autoload fails.
As public is the working directory, you can add Model as Library only with using an absolute path, as Axel does not support '../', it does only replace './' with '':
if ($baseDir[0] == '/' || $baseDir[1] == ':') $this->baseDir = $baseDir;
else $this->baseDir = getcwd() . DIRECTORY_SEPARATOR . str_replace(['./'], '', $baseDir);