What is 4K passthrough? I see that newer AV receivers support it, but then I also see only one HDMI input that supports HDCP 2.2. Does this mean that 4K passthrough does not need HDCP 2.2? Doesn't 4K require HDMI 2.0 and HDCP 2.2? Does that mean not all of the inputs support "true" 4K?
You're not alone; this is a topic of much confusion. 4K passthrough is an AV receiver's ability to accept a 4K video signal from a source device and pass it through to a 4K display. To pass a copy-protected 4K signal—which will include virtually all commercial 4K/UHD content for the home—the HDMI output of the source device must be connected to an HDMI input on the AVR that supports HDCP 2.2 copy protection. The AVR's HDMI output is then connected to an HDMI input on the display, which must also support HDCP 2.2. In the example you cite, 4K passthrough would only work with the AVR's one HDMI input that supports HDCP 2.2.
These days, it's common for AVRs to have many HDMI inputs, only one or some of which support HDCP 2.2—if there are any at all. When shopping for an AVR, be sure at least one input supports HDCP 2.2; if it has no such inputs, the signal from any upcoming Ultra HD Blu-ray player or 4K/UHD streaming device will not pass through the AVR and on to the display. This does not apply to the streaming apps within the display itself, which do not send any video signals via HDMI to the AVR. (They might send audio via the display's optical output or HDMI Audio Return Channel, but that shouldn't be a problem for HDCP 2.2.)
There are some related issues to consider when shopping for an AVR. For example, HDMI 2.0 can operate at a bitrate of 10.2 or 18 Gbps, and it's usually not clear which bitrate is implemented in any HDMI 2.0-equipped device. At 10.2 Gbps, the connection can carry 2160p at 60 frames per second with no more than 4:2:0 color subsampling and 8-bit resolution, which is insufficient for high dynamic range (HDR). The lower bitrate can convey up to 12-bit resolution and 4:2:2 color subsampling at 30 and 24 frames per second, which is sufficient for HDR at these lower frame rates. Still, I strongly recommend getting an AVR that operates at the higher HDMI bitrate of 18 Gbps if possible, because that is more future-proof than the lower bitrate.
Another issue is HDR itself, which I believe is critically important for the future of video entertainment. To convey an HDR signal from a source device—say, an Ultra HD Blu-ray player or 4K/UHD streaming device—through an AVR and on to the display, all devices must have HDMI 2.0a connections. The "a" signifies that the connection can convey HDR metadata, which is required for an HDR display to properly reproduce HDR images.
However, keep in mind that just because HDMI 2.0a specifies this capability, that doesn't mean a hardware manufacturer must implement it, even if the connection is identified as HDMI 2.0a. So be very careful about determining exactly what capabilities are actually implemented in a given product, not just what HDMI version number it uses. The version number represents the set of capabilities that can be implemented, not which ones are actually implemented.
If you've got an AV question, please send it to Scott Wilkinson (firstname.lastname@example.org) or Mark Henninger (imagic, email@example.com) via PM or email.