On the SitePoint PHP Blog, there's a tutorial posted from editor Bruno Skvorc giving an introduction to Symfony Flex and how it is "paving the way" to a more performant future for Symfony.
Symfony Flex is a modern replacement for the Symfony Installer and not the name of the next Symfony version.
Internally, Symfony Flex is a Composer plugin that modifies the behavior of the require and update commands. When installing or updating dependencies in a Flex-enabled application, Symfony can perform tasks before and after the execution of Composer tasks.
The new Symfony will be called just Symfony 4, and while this tutorial will deal only with the Flex tool, it will mention some Symfony 4 upgrades as well.
The tutorial starts with some of the basics about Flex including its current development status and what kinds of things have changed from previous Symfony setups. It then walks you through the creation of a new Flex application including the bootstrapping of the application and the setup and use of application bundles.