**Resolved, details below the issue.
I have an LG 27un850-w display (supports up to 4K 60Hz).
Got a Chromecast w/ Google TV (4K model) today... Hooked it up, disabled HDMI ULTRA HD Deep Color in the monitor's settings to get it working... So far so good... Got in, updated the firmware for the unit & remote, and when I went to set the display settings, it's offering up to 4k 30Hz as the top resolution, which is unfortunate.
Any ideas on ways to get full 60Hz refresh rate going at 4K? Rest of the internet makes it look like the Chromecast should be able to handle it.
I had to mess with the Chromecast's color format settings (I set it to YCbCr 4:2:2 8-bit), then in the monitor settings, I had to re-enable the HDMI ULTRA HD Deep Color setting. Once I did that, the monitor restarted, and Chromecast resolution options then offered up to 60hz at 4K. Offers more / better color options, too!
Weird circular set of things I had to do do get the device to work right with this monitor, but now everything looks perfect best I can tell.