Symfony 4 was released on November 30th.
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New in Symfony 3.3: WebLink component

Symfony 3.3 will include a new component called WebLink to manage links between resources; implementing features such as preloading and prefetching.

New in Symfony 3.3: SecurityBundle improvements

In Symfony 3.3, the SecuritBundle includes improvements such as a smarter security:encode-password command and a more robust logout URL generation.

New in Symfony 3.3: Redesigned exception pages

Symfony 3.3 has redesigned the exception pages to modernize them and match their design with the toolbar and the profiler.

New in Symfony 3.3: Faster routing

In Symfony 3.3 we optimized the performance of the routing component to make your applications faster, specially when they define a lot of routes.

New in Symfony 3.3: A simpler way to get the project root directory

Symfony 3.3 defines a new Kernel::getProjectDir() to replace Kernel::getRootDir() as the way to find the root project directory.

New in Symfony 3.3: Improved flash messages

In Symfony 3.3, it's easier to work with flash messages in templates. In addition, the session no longer starts automatically when checking if there are flash messages.

New in Symfony 3.3: Workflow improvements

In Symfony 3.3, the Workflow component will include lots of useful new features.

New in Symfony 3.3: Deprecated the special SYMFONY__ environment variables

In Symfony 3.3, the special SYMFONY__ environment variables have been deprecated and they will no longer work on Symfony 4.0. Use regular environment variables instead.

New in Symfony 3.3: "about" command

Symfony 3.3 adds a new command called "about" to display useful information about your Symfony application and your PHP environment.

New in Symfony 3.3: Manifest-based asset versioning

Symfony 3.3 adds a new versioning strategy based on JSON file manifests, which is used by popular tools such as Webpack.