Symfony 4.1 introduces a new utility class to work with HTTP headers, adds support for default values in Accept headers and it allows to bypass HTTP headers when submitting forms in tests.
In Symfony 4.1 the serializer can normalize validation errors automatically, it can define default constructor arguments when denormalizing contents, it allows to use custom max depth handlers and it ignores XML comments by default.
In Symfony 4.1 it's easier to make Ajax requests in functional tests and the Ajax panel in the debug toolbar has been improved.
In Symfony 4.1 you can ignore the log messages related to specific HTTP status codes so your log files are not polluted with irrelevant messages.
Symfony Security Monitoring is a new service that alerts you whenever a security vulnerability is discovered and affects to your projects. It's also a great way to contribute to the development of Symfony.