The short explanation
4K, properly Ultra HD, packs four Full HD frames into one. Cinema 4K (DCI) is slightly wider at 4096 pixels, which is why some specification sheets list both.
The resolution alone tells you little. 4K30 in 4:2:0 is undemanding. 4K60 in 4:4:4 at 8-bit needs a 600MHz TMDS clock — 18Gbps, the ceiling of the older HDMI generation. 4K120 needs FRL and HDMI 2.1.
Where you run into it
This is where most real-world HDMI complaints live. “My TV does 4K but not 4K 4:4:4” is usually a settings problem, not a hardware one: many UHD televisions need an explicit option enabled per input before they will accept a full-bandwidth 4K signal, and often only some inputs support it.
The rest of the time it is the capability data. A chain that reports 4K but not the 4:4:4 variant will get 4:2:0 from the source, and the difference shows up immediately on PC text.
How the DR HDMI 8K handles it
The DR HDMI 8K carries a large part of its EDID library at 4K: 4K120 in 4:4:4 and 4:2:0, 4K60 and 4K50 at 600MHz, 4K60 4:2:0 at 12-bit and 8-bit, 4K30 in 4:4:4 and RGB, and 4K24 4:2:2 12-bit. Presenting one of those is the quickest way to find out whether a missing 4K mode is a chain problem or a display setting.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.