Problem/Motivation
Symfony 4 does not contain hard breaking changes compared to Symfony 3 - at least for the components that Drupal core uses and in the way that we use them. Therefore, once the issues that do affect us are fixed (mainly the removal of Symfony's classloader), it should be possible to allow sites to run Symfony 4.
We'll still want to package core with Symfony 3, Symfony 4 would only be available installing with composer.