According to the latest information given by Friedens and people from the management of Hyperion, the initial final release candidate version is planned on the end of 2006. But release of AmigaOS 4.0 depends on hardware available which runs the system. So, if until December there is no new hardware for AmigaOS 4.0, the system will not be released. Yesterday in Italy a new potential hardware was presented. It is called Samantha. But it is hard to tell if it is going to be available in mass production till December.
Take a look here to verify what I wrote:
yesterday's chat with Thomas Frieden
new hardware specs
chat with Friedens last Saturday