In Symfony 4.1, the commands passed to the process component can contain placeholders whose values are defined at runtime or obtained through environment variables.
In Symfony 4.1 you can configure the cost of hashing a password with the Argon2i algorithm thanks to three new options called memory_cost, time_cost and threads.
In Symfony 4.1, the bundle notation has been deprecated in favor of regular PHP namespaces.
Symfony 4.1 adds support for making 307 and 308 redirections, which are similar to 301 and 302 redirections but they don't allow changing the request method.
In Symfony 4.1, the Url, Locale and Expression constraints have been improved with new config options.
In Symfony 4.1 applications, matching a URL is 77 times faster than in previous Symfony versions, becoming the fastest router available in PHP.
Symfony applications starting from 3.4 are now compatible with "reproducible builds", a set of practices to ensure the same compilation result when the source code doesn't change.
Symfony 4.1 added support for DateTimeImmutable objects in every form type related to date and time.
In Symfony 4.1 you can get all container parameters in a service injecting one of its constructor arguments with the ParameterBagInterface.