I know this is old but I got an Onkyo TX-NR6100 Sub-out working with Hyperion NG while keeping 4K output to my TV.
Parts List from Amazon (CA):
I set the 5-scaler dip switch positions to all "On" (down position).
PIN 1: On = Test Mode (send HDCP is available)
PIN 2: On = Scale output to 1080p
PIN 3 - 5: On = EDID to AVR is 4K60Hz (444) 8-Ch HDR
AVR Sub Out -->EDID Scaler In --> EDID Scaler Out -> USB Capture Card --> RPi 3 w/Hyperion
I set the EDID setting to max 4K so the TV will be the lowest factor when the AVR decides which max settings to use.