You are building a really strong single-CPU system and it should have no trouble at all with GH4 files if your system is set up properly. You will read a lot on the www regarding how demanding 4K is to edit, however most of those users are referring to RED 4k and other more professional media formats that are brutal on a system.
For Premiere Pro:
- CPU/motherboard/32GB RAM - just fine; get a good CPU cooler and do a minor OC and it will be even better
- 850 Pro boot drive - 256GB model would be fine for Adobe, only get the larger one if you need it for your game dev. work
- dual GTX 880 - way overkill for this system, but again you may need this for your work; NVIDIA GTX are way better for PPro than ATI cards, so if you have any performance issues using the 7970's, those issues will probably all go away when you go GTX
- drives - you only need the 3x or 4x 4TB in RAID 0 for PPro; it will be plenty fast and the 32GB of RAM will help gloss over any HD latency
- drives - uber performance option - multiple SSDs in RAID 0 perform way better than single large drives (so long as you get SSDs that do RAID 0 well); I already said you do not NEED more drive power for PPro, but if you are still wanting a high-speed data work space think 4x Crucial 256GB SSD in place of the Samsung 1TB EVO
As Jim says the Samsung EVO drives are lousy when you are writing to them with uncompromisable video data. Read performance is OK
Be careful when you choose the USB3 Flash drives they vary in performance from lousy to very good. Currently, with lots of reviewing and testing on my part I like the PNY Turbo 128 GB or its 256GB big brother.
Assuming you mean 'incompressible', I don't think that really affects the EVOs much - I was under the impression that's primarily a problem on Sandforce controllers. In any case I didn't realize the Crucial MX100 series were so fast and so cheap, so I'll probably RAID those up like Jim suggested.
As far as the USB3 drive, I'm planning to get a multi-terabyte USB3/TB magnetic external, not a flash drive.
The Crucial M550 drives are what I would suggest on the Crucial side. The Samsung 850 Pro drives are far better than the Evo and released at a very reasonable price. The caching and performance of the EVO drives is poor especially if they are raided. I would not suggest Evo's for anything but OS.