ZBrush 4.0 has been released by Pixologic for both the Mac and PC as of today. All customers who have purchased previous versions of ZBrush are eligible for a free upgrade and should be receiving an email directly from Pixologic. If you don't receive yours, you can call us at 310-450-4000 and we'll get your ZBrush 4.0 license for you.
If you are a new ZBrush user, as of right now, your purchase is available as a download only, and boxes will be shipped at yet undetermined later date.
Also being released is a special anniversary edition of Zbrush that comes in a special tin case (pictured). Details on that coming soon.