Hi Brian,
I am not really sure when is the date but the news is that beta is very soon. But I should note though that we have used OpenACS 4 even before its alpa stage. We have a couple of production sites using just the CVS code. We also have 1 site that is heavily pounded everyday. Since you will be just starting you will mostly have little problems even in pre beta stage.
I suggest that you can start developing on it as long as you are confortable using CVS and changing/upgrading the data model. Get the feel of OpenACS 4, since the documentation is still being done.