Install
Works with Drupal: 7.xUsing dev releases is not recommended, except for testing.
Using Composer to manage Drupal site dependencies
Downloads
Release notes
The 4.x branch represents substantial change from the 3.x branch.
The 3.x branch has quite a bit of code that exists purely for historical reasons. The underlying Facebook API have been modernized, but the drupal module has evolved more slowly.
The 4.x branch is essentially a complete rewrite. It uses only the more modern Facebook APIs. It removes dependencies on Facebook PHP and Javascript code. And that in and of itself is a major gain.
Don't expect your existing site which uses 3.x to upgrade seamlessly to 4.x. At the moment, the features are not identical and possibly they never will be.