So after owning this TV for a couple of years, I finally had a reason to determine whether or not I could see a Youtube movie rental in 4K UHD. Easy question to find a definitive answer to ... I thought. No.
Everything I see about HDCP 2.2 and 4K UHD viewing only speaks to connections via HDMI. However, nothing speaks to the ability of the Youtube app on the Roku TV being able to play it.
TV supports it so I am pretty sure the answer is yes, I can see 4K UHD content on the builtin Youtube player. However, it would be nice to "know" before I spend the money on a 4K UHD rental.