As I tried (but maybe failed) to make clear in the release notes for v4.0, the core functionality hasn’t changed and will continue to work in the same way – there are even some new capabilities. If that’s all you want just dismiss the Freemius message and carry on as before.
However, if you want a UI to manage the settings, a log of the events so you can see what’s going on and from which country (with a couple of dashboard widgets), and some reporting (more to come), one of the Premium versions might be for you.
Next is better multi-site support, and a REST API so you can manage all your installs and sites from one place (also Premium features).
Again, if you’re happy with things as they were in 3.x there’s nothing that will force you to change or pay any money – everything will carry on working exactly as it did before.