Larry has 12-year-old florescent backlit TV and it's time to upgrade to 4K. It's in a surround sound system with a Denon receiver and he would rather not upgrade that as well. Leo says there is an "upgrade cascade" that happens when there is a home theater system. If an AV system drives video, then users have to upgrade it. But if it's just running the audio, they'll be fine. Just use the optical out on a new TV. Upgrade the ROKU box to 4K capable. But what's even more important is the HDR support 4K brings. Leo adds to take the Denon and plug it into the 4K TV along with a device and see what the TV is seeing. It'll likely be 1080p60, not 4K.
From Mike in the chatroom - HDMI 2.0/HDCP, which is supported by most current products, will do 4K.