Hm interesting. @parndt, I'm not sure if I understand your statement. In Rails 4, there shouldn't be an assets group line at all in your Gemfile to begin with:
http://guides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-3-2-to-rails-4-0-gemfile

When upgrading from Rails 3.2 to Rails 4 the default way, you'll have to remove the assets group line altogether in your Gemfile, and modify the `config/application.rb` file in your Rails app according to the link above.

If you're creating a new Rails 4 app from scratch, it comes without the assets group line in the Gemfile by default.