Igor, very little activity is taking place on older versions of OpenACS
and all the effort towards perfection and polish are taking place
in the 4.x version. It is not a "transition copy" since there are
no plans to do anything other than make further evolutionary
changes to the code base (all of which should be upgradable). Furthermore, 4.x has very significant advantages
over the 3.x version in terms of design, modularity, upgradability, and
functionality. So, while you can build a site (and a business maybe)
on 3.x, 4.x is almost certainly the prefered choice.
Also, in the next
month or so 4.6 should be released and I think you will find that
quite a few of the 4.5 bugs have finally been ironed out.