Selecting a CMS for a project is never an easy task and get an updated one could help you narrow down the choices.
This is a list of Rails content management systems that support Rails 4 (or will support it soon).
But more are coming…
BrowserCMS is going to support it https://github.com/browsermedia/browsercms/wiki/Roadmap
…and LocomotiveCMS should be ready this spring https://github.com/locomotivecms/engine
Radiant and Refinery are working on it: