On the surface they do appear to be the same but the Sky Lab has apparently been completely rewritten (not just the UI but the code that does the hard work as well). I hadn't got into Bryce3D that deeply before upgrading to 4 but the main things I noticed were the improved render speeds and when a Poser figure was imported the meshes were named (ie head1 instead of meshX). Initially I was slightly disappointed with the upgrade as it didn't appear to have many obvious new features (certainly nothing as spectacular as the upgrade from Poser 3 to 4) but now I've dug deeper into it's capabilities I would definitely recommend you go for the upgrade. -Tony