This page has dates for all of the pre-release updates of OS4:

http://www.gregdonner.org/workbench/wb_40.html

As others have mentioned the non-availability of hardware is the only factor preventing release.

If I remember rightly Hyperion stated around the time of Update 4 that if hardware was available this would have been the final release.

Since then they have fixed more bugs and added more features, judging by some of the reports available from beta testers and shows, so the final release will include stuff not currently available to users of Update 4.

Hyperion at one point said that they hoped to have a final version ready for release by the end of the year, although there have been no public hardware developments that would make this possible. (It is, of course, impossible to rule out any non-public developments that we aren't aware of)

More recently developers from Hyperion have hinted at some kind of announcement happening this month, so there may be further news soon.

One small issue I found while registering - once I activated my account, I tried to login on the page I was redirected to. It did log me in, but I was then taken to the activation page which told me I was already activated, and then redirected to itself.