The secret cookie store was recently removed in Rails 4. See Changelog.
In order to get your app working again, replace the line config.session_store :encrypted_cookie_store with config.session_store :cookie_store in config/initializers/session_store.rb. The store will be automatically encrypted.
If unsure. Create a new Rails 4 app (rails new app_name --pre) and have a look at the initializers.
For details see the according commit