What is HDCP?

HDCP is copy protection built into HDMI: the streaming device and screen must cryptographically agree before protected video plays. 4K content requires HDCP 2.2 or newer.

It's why a 4K stick on an older TV, monitor or HDMI splitter can show a black screen or drop to HD. The fix is using an HDCP 2.2-capable port (often labelled) and a decent High Speed cable.