I just published a leanpub book on how to upgrade your Rails 3
to Rails 4. It covers all new features introduced in Rails 4 and
the deprecated features as well. You can follow along with an included
upgrade checklist to see if you upgraded all breaking changes from Rails
to Rails 4.
You can get it over here: Upgrade to Rails 4 by Philip De Smedt [Leanpub PDF/iPad/Kindle].
and suggestions are very welcome. Thanks!