Fabien Potencier has a quick post to his site that may be of interests to the Symfony developers out there related to directory updates coming in Symfony 4:

After a long discussion in the community, the directory structure has been slightly changed.

etc/ was renamed to config/, and web/ to public/. My blog posts about Symfony 4 have just been updated to reflect these changes.

He also includes instructions about how, if you're already running a Symfony Flex application, you can make these changes manually including updating the Flex version.