Symfony 3.4 will include support for HTTP Immutable Responses thanks to the new immutable option of the Cache-Control directive.
Symfony 3.4 adds support for validation groups in the Valid constraint, so you can partially validate the embedded objects.
Symfony 3.4 improves the exceptions displayed in the console to include by default the exact file and line number where the exception occurred.
Symfony 3.4 improves the user impersonation feature of the Symfony component to make it easier to debug.
In Symfony 3.4, security listeners are lazy loaded to improve performance. In addition, the Symfony Profiler has been updated to show all the information about security listeners.
Symfony 3.4 adds support for XLIFF 2.0 notes, which allow to store end user comments and annotations about translations.
In Symfony 3.4, the @Route annotation can define a prefix for the names of all the routes defined in the same controller class.
In Symfony 3.4, console commands can be lazy-loaded, so the application doesn't have to instantiate all of them when running any command.
In Symfony 3.4, the Stopwatch component allows to reset all its time measures and removes the sub-millisecond restriction to allow time measures with arbitrary precision.
In Symfony 3.4, the web debug toolbar and the profiler will include a new panel to collect information about validator calls.