No 4K TV here but I can say that V6 has improved the picture clarity on my 1080P TV by upscaling broadcast channels, something which TiVo can't do.
From other threads I have seen, it is important, while there is no broadcast 4K material, to ensure that V6 allows your TV to do the upscaling to 4K of broadcast channels as your TV may do a better job than V6. I believe this would be done by not ticking the 2160P box in video settings, while leaving the 2160P passthru ticked to allow Netflix and YouTube 4K material straight through.
I have 2 V6 boxes both connected to 4K Televisions, I have one set up with a powerline to my superhub 2 and the other connects via wi-fi.
whilst Ernie is right that there is no actual 4K broadcasts the picture quality is excellent on the 4K material that Netflix has, I'm sure as you've probally seen in other posts connecting to Netflix is also so much faster than the standard TiVo box.
with regard to settings the boxes are set 216p on full screen, in fairness I think the box automatically defaulted to that setting so never seen the need to change it.
Picture quality very crisp and clear. Video settings on v6 2160. No problems with picture quality at all. Box works extremely well with my 4k tv. The only thing I had to adjust was colour and brightness.
@Blenky. As you know, I don't have a 4K TV but I just loaded iPlayer on my V6 and the first screen I reached invited me to try their trial. So looks like that trial would be available to test out the V6's capability.
I don't think the HDR level of V6 firmware is available yet.