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
Regards,
Jim