In an article on his Medium blog, Jerzy Zawadzki wrote about the most important changes made in Symfony 4.

Internally, Symfony 4.0 is “just” Symfony 3.4 with removed depracations. But from outside there is a big leap forward. Most changes (from the installation process, directory structe through using bundles, to coding itself) were made to improve Developer Experience with the framework. Such system like Symfony, which can be used to create web apps as easily as to build other frameworks on top of it, must be complicated. But, as Symfony proves in new version, this complexity may be ‘hidden’ from the developer eyes.

https://medium.com/@zawadzki.jerzy/symfony-4-new-hope-dbf99dde91d8