My old system had very similar specs, and to edit native UHD 25p footage was possible but on the limit. i used mp4 as record format. I overclocked the 2600k to something about 4.2GHz. And I optimised dynamic ram preview and used the GPU acceleration for the preview. I never used proxys. And XAVC would have a tougher playback behaviour then the GH4 mp4 files I think.

Sorry, pardon my ignorance.
How do I do a XAVC proxy render?
Is it a conversion?
Something that can be done inside Vegas or is third party software required?
Beginning to regret not shooting this project in 1080

Solution could be to batch render to 1080.
Anyone have a workflow for that?
Technical moron here, as you can tell.

Solution could be to batch render to 1080.

You really don't want to do that at the beginning if your final delivery will be 1080 unless you intend to use that as a proxy. The workflow for using a proxy is really quite simple. First, render all of your footage to a lower resolution set of files, referred to as your proxies. If your system is really "weak", you could render to something like HD EX 1280 x 720, making sure you change the frame rate to 30P, assuming you are in NTSC land. Then replace the original files, with the proxy files you just rendered. You should also change your project settings to reflect the proxies. At that point, your preview should be pretty good and you can then do your editing. Once finished with your editing, you simply replace the proxy files with the original 4K footage files, change your project settings back to the original and then do a final render to a delivery format of your choosing. To do batch replacement of original files with proxies and vice-versa, there are freeware scripts that can be used or scripting extensions like Vegasaur. On the other hand, if you only have a few files, you can do it manually just as easy.

wwaag

Sure, but to ignore the fact that other people are able to edit with the same processor the footage native is also funny.