I'm a bit late in taking note of this – ArtRage 4 released back in February. Look, ArtRage is the best value you are going for a painting program you are going to get on Windows or Mac (it runs on both). I've been recommending it for years, and ArtRage 4,0 still my top pick.
From a feature perspective there are a couple of nice improvements since ArtRage 3.5.5 but nothing major. Having said that let's look more closely.
The Application icon has received a minor tweak. The screenshot below is from the Windows 8 Start Screen with ArtRage Studio Pro 3.5.5 on the left and ArtRage 4 on the right. Apparently they've dropped the “Studio Pro” from the name.
Overall Application: ArtRage Studio Pro 3.5.5
Overall Application: ArtRage 4
The overall interface hasn't changed. You may notice though that some icons look a bit different.
However ArtRage 4 does have a special interface activated via View > Workbench Mode
The Workbench Mode is a minimal interface with a thin strip of tools at the top.
You can access all the brushes by click on the crush icon.
Clicking on color square yields special color picker.
I was already very please with ArtRage's minimal UI and easy-to-access fullscreen mode, so this Workbench Mode isn't that interesting to me.
If you want to see it in action, there's a YouTube video available of someone walking through some more UI changes and new feature