For those looking to make the move up to Symfony 4 and Flex the Knp University site has a new post that sheds some light on the composer.json configuration for Flex and how to use it to install a complete Symfony 4 application.
We need to make our composer.json file look like the one from symfony/skeleton. Actually, go to "Releases", find the latest release, and then click to browse the files. Now we can see the stable composer.json contents.
So... yea, this one file is all you need to start a new project. That's crazy! Flex builds the project structure around it.
The article then talks about the major changes including dropping the
symfony/symfony package, how to synchronize the rest of the
composer.json and autoloading updates. It ends with a brief mention of how to then scaffold out the full application structure - one command, a Composer update.