Hi Fred
I'm currently in the process of putting a Rails 4 app with Plutus into production. The changes to the rails 4 branch were minimal and consisted of updating a couple of deprecated finders and removing the attr_accessible from the models which was pretty much unneeded.
In general though, Plutus is still a pre 1.0 release, so things may be changing substantially in the future. But I don't see any reason why the current rails 4 branch would have any problems in production.
-- Mike