This post is just in case you have a similar setup and the solution I found. I'm running i7700/4200 32 gig Ram /AMD480 8 gig / SSD /fast spinners, 4K 50" Tv and widescreen 2180 monitor. I was having lots of issues when working and having dual monitors working. Finally I tried one last thing, I switched which monitor is primary in Vegas. With the 4K TV as #1 and the widescreen monitor as #2 no issues with fairly complex Multi-track 4K projects.