B4.1 added some much need volumetric presets that Ken Musgrave created. During beta testing we also finally added the true volumetric mode (called Basic shading). B4.1 also added a mode called sky integration, which allows volumetric clouds to blend nicely with existing atmospheric effects. Of course all of these features are also in Bryce5. Volumetric slab clouds take forever to render - sometimes it is better to make a few foreground free-standing clouds with spheres. During creation of volumes - the key it is try and get the effect on the rabbit rather than the tortoise side of the render slider. Good luck - Scott : )