- https://www.ombulabs.com/blog/rails/tips-for-upgrading-rails-3-2-to-4.html
- https://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-3-2-to-rails-4-0
- https://github.com/alindeman/upgradingtorails4
- https://github.com/alindeman/rails4_upgrade
- https://thomasleecopeland.com/2015/08/06/running-rails-update.html
- https://mensfeld.pl/2013/06/upgrading-to-rails-4-0-from-rails-3-2-test-case-part-i-preparations-configuration-gems/
- https://www.ombulabs.com/blog/rails/upgrades/upgrade-rails-from-3-2-to-4-0.html
- https://www.webascender.com/blog/crucial-steps-when-upgrading-an-application-from-rails-3-to-rails-4/
- http://www.rails-upgrade-checklist.com/#4.0
- http://blog.barbershoplabs.com/blog/2013/02/27/upgrading-from-rails-32-to-rails-40
- http://railscasts.com/episodes/415-upgrading-to-rails-4
https://weblog.rubyonrails.org/2013/6/25/Rails-4-0-final/
- https://github.com/rails/rails/blob/v4.0.0/actionpack/CHANGELOG.md
- https://github.com/rails/rails/blob/v4.0.0/activemodel/CHANGELOG.md
- https://github.com/rails/rails/blob/v4.0.0/activerecord/CHANGELOG.md
- https://github.com/rails/rails/blob/v4.0.0/activesupport/CHANGELOG.md
- https://github.com/rails/rails/blob/v4.0.0/railties/CHANGELOG.md