Backwards compatibility

Hi,

Yes it's currently compatible with php 5.4 and WP 5.1.
Though it's not recommended at all, as this Php version is not supported anymore by the php team.

Wordpress is dropping support of outdated php versions for the good of all users.
The current WP Media Folder version will work with your configuration but we will drop the support in some time and you will not be able to update the plugin anymore with this php version.

Best regards