And a pretty solid update too! Pixologic servers are getting hammered at the moment, but I managed to get the download and activation in before the slowdown. Shame I have to be at work now and can't play with it just yet. 
Link
New Feature list and improvements:
ZProjects
* Save all loaded tools, document size, light, material, timeline and render settings in one click.
* ZBrush 4 even includes sample projects to help you discover new features.
Best Preview Render
* Renders Shadows, Ambient Occlusion, Fibers, SSS and Transparency with advanced anti-aliasing.
* Multi-pass rendering with Shadow, Color, Ambient Occlusion, Mask and Depth as separate maps.
* Render for still images or animations.
Project presentation
* Timeline for more than just simple turntables and animations.
* Keyframe management: copy, move, etc.
* Audio support with color beats for manual animation synchronization.
* Fade in/out and cut keyframes for nice transitions.
* Ease in/out keyframes.
* MDD animation file format support for exporting and importing animation.
* Animation-capable layers, Tools, ZSpheres, background and more.
Concept creation
* Shadow Box for free-form creation.
* Move Elastic Brush to create stretch-free extrusions.
* Reproject improved for more finely tuned control of the results.
* Mannequin for quick scene setup.
* TransPose units to help you in proportions and sizes.
Hard Sculpting
* Clip brushes for slicing your models.
* By Radius and By Polygroups modes for the Clip brushes.
* Shadow Box to create original sculpting bases, combined with radial symmetry.
* New Strokes: Circle, Square, Curves.
Major brushes:
* MatchMaker, the perfect brush to deform your mesh to conform to other 3D models.
* Deco brushes, turn, spin - perfect for creative patterns!
* Clay Buildup will be your new Clay brush for the rough sculpting.
* Move Elastic, Move Topo, Move Parts: Stretch and deform with no constraints.
Strokes:
* Roll mode improved with the Roll distance.
* New lasso/marquee selection mode, circle by center, perfect circle, curve for masking and hiding.
* Marquee, Lasso and Circle strokes working with symmetry for hiding, masking and slicing.
Pipeline and Productivity
* GoZ with Autodesk Maya (2008, 2009, 2010, 2011), Autodesk 3DSMax (2009, 2010, 2011), Luxology modo 401 (Sp2 to sp5), Cinema 4D R11 and R11.5
* GoZ SDK for integrating GoZ in additional software packages.
* Xpose to explode all SubTools, making it easy to work on a hidden one or create stunning animated effects.
* SubTool Duplicate and Insert, List All + ABC, Merge All for more flexibility when working on complex models.
* Solo mode to quickly isolate the current SubTool.
3D Painting and Materials
* PolyPaint Layers for more freedom in 3D painting.
* SpotLight to edit textures on the fly and then use them to paint by projection.
* Gradient transparency for anti-aliased transparency in PolyPainting.
* New shaders: Fast Overlay and Fresnel Overlay for more effects in your materials.
* New Fresnel and SSS settings for more realistic materials!
* New Blinn Specular added to the default shader for a better skin specular effect.
* Large number of blending modes between materials: Add, Subtract, difference, darken, etc.
Misc
* Topology auto masking to protect closed areas which are not directly connected.
* Elasticity settings to stretch your model without stretching the topology.
* Orientation settings: rotate your alphas while sculpting.
* Tile alphas for more creativity with brushes.
* Non-Square alphas are supported, avoiding distortions.
* Contact support for SubTools: deform multiple SubTools while maintaining your defined contact points between them!
* Relax Deformation: relax your topology without altering the shape.
* Inflate Balloon Deformation for a variation of the relax deformation.
* Deformations are now in real-time when moving the sliders, giving you live feedback.
* New File menu to manage all content.
Plugins
* Decimation Master is now 64bits.
* Original size is kept when exporting with 3D Print Exporter.
* Unwrap all SubTools at once with UVMaster.
* New Multi-map exporter for fast and easy creation of many maps at once including: normal, displacement, ambient occlusion, etc.
* Various additions in SubTool Master and TransPose Master.
Link
New Feature list and improvements:
ZProjects
* Save all loaded tools, document size, light, material, timeline and render settings in one click.
* ZBrush 4 even includes sample projects to help you discover new features.
Best Preview Render
* Renders Shadows, Ambient Occlusion, Fibers, SSS and Transparency with advanced anti-aliasing.
* Multi-pass rendering with Shadow, Color, Ambient Occlusion, Mask and Depth as separate maps.
* Render for still images or animations.
Project presentation
* Timeline for more than just simple turntables and animations.
* Keyframe management: copy, move, etc.
* Audio support with color beats for manual animation synchronization.
* Fade in/out and cut keyframes for nice transitions.
* Ease in/out keyframes.
* MDD animation file format support for exporting and importing animation.
* Animation-capable layers, Tools, ZSpheres, background and more.
Concept creation
* Shadow Box for free-form creation.
* Move Elastic Brush to create stretch-free extrusions.
* Reproject improved for more finely tuned control of the results.
* Mannequin for quick scene setup.
* TransPose units to help you in proportions and sizes.
Hard Sculpting
* Clip brushes for slicing your models.
* By Radius and By Polygroups modes for the Clip brushes.
* Shadow Box to create original sculpting bases, combined with radial symmetry.
* New Strokes: Circle, Square, Curves.
Major brushes:
* MatchMaker, the perfect brush to deform your mesh to conform to other 3D models.
* Deco brushes, turn, spin - perfect for creative patterns!
* Clay Buildup will be your new Clay brush for the rough sculpting.
* Move Elastic, Move Topo, Move Parts: Stretch and deform with no constraints.
Strokes:
* Roll mode improved with the Roll distance.
* New lasso/marquee selection mode, circle by center, perfect circle, curve for masking and hiding.
* Marquee, Lasso and Circle strokes working with symmetry for hiding, masking and slicing.
Pipeline and Productivity
* GoZ with Autodesk Maya (2008, 2009, 2010, 2011), Autodesk 3DSMax (2009, 2010, 2011), Luxology modo 401 (Sp2 to sp5), Cinema 4D R11 and R11.5
* GoZ SDK for integrating GoZ in additional software packages.
* Xpose to explode all SubTools, making it easy to work on a hidden one or create stunning animated effects.
* SubTool Duplicate and Insert, List All + ABC, Merge All for more flexibility when working on complex models.
* Solo mode to quickly isolate the current SubTool.
3D Painting and Materials
* PolyPaint Layers for more freedom in 3D painting.
* SpotLight to edit textures on the fly and then use them to paint by projection.
* Gradient transparency for anti-aliased transparency in PolyPainting.
* New shaders: Fast Overlay and Fresnel Overlay for more effects in your materials.
* New Fresnel and SSS settings for more realistic materials!
* New Blinn Specular added to the default shader for a better skin specular effect.
* Large number of blending modes between materials: Add, Subtract, difference, darken, etc.
Misc
* Topology auto masking to protect closed areas which are not directly connected.
* Elasticity settings to stretch your model without stretching the topology.
* Orientation settings: rotate your alphas while sculpting.
* Tile alphas for more creativity with brushes.
* Non-Square alphas are supported, avoiding distortions.
* Contact support for SubTools: deform multiple SubTools while maintaining your defined contact points between them!
* Relax Deformation: relax your topology without altering the shape.
* Inflate Balloon Deformation for a variation of the relax deformation.
* Deformations are now in real-time when moving the sliders, giving you live feedback.
* New File menu to manage all content.
Plugins
* Decimation Master is now 64bits.
* Original size is kept when exporting with 3D Print Exporter.
* Unwrap all SubTools at once with UVMaster.
* New Multi-map exporter for fast and easy creation of many maps at once including: normal, displacement, ambient occlusion, etc.
* Various additions in SubTool Master and TransPose Master.
