Does request.body require rewind before read with Rails 4 and content-type application/json or is there a new approach?
This is indeed a bug. The fix has been integrated into master and should presumably pop up in 4.0.1.
It looks like it might be a bug in ActionDispatch::ParamsParser.Putting the following in config/application.rb seemed to fix it
config.middleware.delete ActionDispatch::ParamsParser