Good overview of changes in Rails 4.0:

• Rails.queue
• strong_parameters
• Turbolinks
• Russian Doll Caching (cache_digests)
• ActionController::Live (streaming)
• PATCH verb
• Routing concerns
• Extraction of activeresource
• ActiveRecord::Relation changes and improvements
• Removal of plugin support (not gems; only those that are installed in vendor/plugins)
• Removal of Rails 2 finder syntax
• Deprecation of most dynamic finders
• Deprecation of eager-evaluated scopes

Link: Rails 4.0 Whirlwind Tour via highgroove.com