Convert Rails 3 attr_accessible mass assignment to Rails 4 Strong Parameters.
Convert config files to Rails 4.
Update to Rails 4 gems. This performs the step that actually installs Rails 4.
Use the newest FactoryGirl features to simplify factories.
Slight cleanup in bounty validation syntax.
Rebuild the database, generating a Rails 4-style schema.rb.
Rewrite some DB queries to cope with ActiveRecord changes in Rails 4.