While it still does have some features only available to premium users, it is much less nagging about the process than their v3 client is known to be. Adamar (PC version) and Kaelten (Mac version as well as the addon OneBag) are the developers behind this new version and have been working with ckknight to help integrate some of its features into WoWAce.com and CurseForge addon development sites (both of which are owned by Curse Gaming). This allows players to report bugs via the new client and have them show up as support tickets in the addon sites for the developers to track and work with.
We've put together a couple quick galleries of what both the Mac (thanks Mike!) and PC versions of the new client look like as well as delving into some of the cool new features that are available with premium membership.
Gallery: Curse Client for Mac 4.0 beta
Gallery: Curse Client v4 for PC