When upgrading from Rails 3 to Rails 4, you need to add a couple of new settings; otherwise, JavaScript and CSS will no longer be minified in production.
I have released the first beta version of Upgrading to Rails 4. Rails 4 is not yet released, but might be soon. There are also certain features you can start using today in Rails 3.2 that have been extracted as gems.
I expect to release new content every 1-2 weeks as Rails 4 continues stabilizing.
Highgroove just released the zonebie gem: it runs your Rails/ActiveSupport tests in a random timezone to prevent bugs where code wrongly depends on a certain zone being set.