I will soon be trying to use 4k mode on Panasonic VX981k. Any advice from your 4k editing on the VX870? You're using the proxy? It takes how long to build the proxy? Any multicam 4k?
I regularly edit 4K MP4 from a DVX 200 and GH4 as multicamera material and have no problem with Vegas 14. I don't pre-render at all and while the preview quality isn't great, it works fine.
I use 2 Gx8's and a Gx85 and the mp4 stuff while only 100 mbit rate imports easily and looks very good, down rez to 1080P and you get amazing quality. I find that when I am doing "lots of stuff" to the video working with proxies is the way to go, of course that could be a limitation of my consumer type computer (I7700/32 Gig Ram/Rx480 8 gig/500 Gig SSD for rendering).
I now have some experience with the Panasonic VX981k. It's one of 4 cameras in an multicam edit, so I let it build the proxy. That takes real time, i.e., if the video is an hour, it takes an hour to build the proxy. This is on a AMD 8350 8 core 4.0GHZ CPU. The video card is Nvidia 750ti, but I don't think it's being used to build the proxy.
The multicam 4 window edit frame rate is low, so I set on GOOD HALF usually, and it's acceptable. It's not much different than if it was HD1080 with the proxy.
I am extremely pleased how radically the cropping can be and still look good! I don't know how much of that is due to the good source from the camera, or how Vegas Pro handles the cropping, but it's great.
In my case, this camera is unattended, so crop/pan is necessary to make it look like it's manned. It's some work to do all the pan/zooming, but the result is great without having another operator.
So with just $700 on sale for the VX981k, I'm in the 4k world now. :-)
Maybe one day I can afford the new i9 processor with 36 streams and a proxy won't be needed.