The vast majority of the material in the course is still completely applicable to Rails 4 and Rails 5.
But I believe there's actually an accidental benefit to this covering an older version of Rails.
In the real world, you won't always to working with the most up-to-date Rails version. For a variety of reasons, many companies still have 'legacy' Rails 3 apps in production.
By knowing about the evolution of Rails, you will gain a better understanding of why particular things are they way there are, e.g. why strong parameters succeeded attr_accessible, and the particular quirks of each release, etc.