The Active Record query interface for the most part abstracts SQL from the developer. However, there is a condition that always requires using pure string conditions in a where clause: <> or !=, depending on the database. Starting in Rails 4, query method not has been added to rectify this.

Since Rails 4 has introduced a Queue into the framework, a primary candidate to take advantage of it was Action Mailer.

Yesterday, David Heinemeier Hansson committed a change to Action Pack that renamed all controller *_filter callback methods to *_action.

Rails 4 has introduced a queue interface, allowing you use ActiveSupport::Queue out of the box or use any queue engine that complies with the interface.

One of the biggest changes coming in Rails 4 is the extraction of certain features and moving them to gems. Moving forward, Active Record observers and Action Controller sweepers are going to be available in the rails-observer gem.