We encourage you to peruse the CHANGELOGs for all the Rails frameworks and delight over the hundreds of improvements we've made to Rails 4.0: Action Pack, Active Model, Active Record, Active Support, Rails. If you'd like to learn more about developing Rails 4 applications, the final version of Agile Web Development with Rails 4 was released today as well.
Source: http://weblog.rubyonrails.org/2013/6/25/Rails-4-0-final/