I have tried the beta 4. It looks nice, however I have noticed that Test, Learn, Match windows behave slightly different than in Stackz 2006. I do not know if this is a bug or not, but I did not like it.
In Stackz 2006 the Test, Learn, Match windows were partly independent of the main window, i.e. they could separately go on top of other applications, had a separate button in a taskbar. I really like it.
In Stackz 07 however this is no longer the case :(.