I have TWO Chromecast 4K w/ GTV and they both do this on different TVs. I'll wake up the device and when I start menu navigation, each time something scrolls or basically any movement happens, the screen shows pixelization mostly across the lower third of the screen. This happens on the home page, in-app, during streaming, and in system menus. Restarting is a temp fix.
Since I have two devices on two televisions, and I've updated and swapped the Chromecasts, I do believe this is a device issue.
Thanks for posting and for being a step ahead of us — I appreciate it! To cover all the bases, please plug in the Chromecast with Google TV (4K) devices on a different HDMI port on your TV and make sure they're plugged in securely. Also, reboot your Wi-Fi router.
If the issue persists, perform a factory reset on your Chromecast with Google TV(4K). Here's a guide: How to factory reset your Chromecast.