I have just noticed something odd with my Prime 4…
When using a USB keyboard plugged into USB 1, the Prime 4 GUI does not respond to some of the keys. For example if I try to search for ‘fred everything’, the e doesn’t work, or the space bar.
I tried another keyboard and it worked normally (same USB socket).
I then plugged the “bad” keyboard into the USB 4 socket on the rear - and it worked normally.
Your FAQ states that a keyboard can be used in any USB socket.
Having now tried the “bad” keyboard back in the USB 1 port, it’s working correctly, as if plugging it into USB 4 somehow “woke up” Engine OS. 
[later] Tried rebooting the Prime 4 and using USB 1 again. Keyboard initially worked, then suddenly started missing keys again. It seems to be related to the OSK.
Just a thought, As the prime 4 is running Linux under the hood its possible the driver did not load correctly the first time the keyboard was dectected on the USB controller.
Was the keyboard already plugged into the USB port before power-up?
I’m thinking maybe the unplugging and plugging (into any USB port) would have caused the prime to re-check it’s ports for a keyboard, in a way that it doesn’t do, for some reason, on power up)
No, plugged in after unit has fully booted.
As I added in a footnote earlier, the random coming and going of functionality seems linked to the OSK.
It may be that it doesn’t work well with two keyboards available to the system simultaneously
Here’s the thing. Denon have stated that we can plug in and use USB keyboards…
Maybe there should be an “Internal / external “ option for keyboards in the preference menu
I will see how my Prime 4 behaves with a USB keyboard tomorrow out of interest
I’d anticipate a whole load of “fun” with assorted usb keyboards. Probably best to avoid keyboards with odd extra keys all over them, and dozens of LED lights, USB hubs, SD card slots, soup makers, milk frothers and all that extraneous crapola
The keyboard I used was plain and simple. A mini keyboard with not even a number pad. One of a pair I used without issue with my old HS5500 players.
KeySonic ACK-3400U