Some info from their promo posting:
Here are some of the highlights of the 4.0 update:
Improved material features with dialogue box controls for fine-tuning your materials on all diffuse texture layers, skin and HD layer materials.
Added a second layer to your HD lips and separated eyes as independent entities that you can edit and hide individually or together for more versatility and creativity.
Updated Style HUD with slots to save your favourite eye looks and revamped main HUD with an extra mode option and a transparent/opaque minimized icon to customize your experience according to your needs and preferences.
There is a lot more that has been don under-the-HUD to speed up and better your experience but the list is too long to put in this short announcement.