Just FYI, you can use Travis to test against various Gemfiles that specify particular versions of dependencies. For example, we do this with CarrierWave:
https://github.com/jnicklas/carrierwave/blob/master/.travis.yml

Rails 4 (master) Gemfile:
https://github.com/jnicklas/carrierwave/blob/master/gemfiles/rails_master.gemfile

I can try to work up a PR that does this for you, but it may take a few commits of tweaking before it runs clean on Travis.

24,420,229 jobs processed using Sidekiq 2.9.0 and Rails 4.0.0 beta 1 in production. Using a combination of perform_async, delay and push_bulk for queueing jobs. Honeybadger integration also works.

I'm using it in development on Rails 4.0.0.rc1, and things generally seem to work well. The only thing I'm having trouble with is exception notifications being dispatched in workers (via exception_notification gem), but that may not be related to Rails 4.