Try connecting your Oppo or PS4 directly to your display (completely bypass the Marantz receiver). See what 4K modes will work. It could be similar to your Oppo/PS4 not being able to query the final display device for HDCP2.2 / 4K compatibility, like this Oppo bug that was fixed:
- Resolved an HDCP 2.2 failure when connecting to an HDCP 2.2 receiver and an older, non-HDCP 2.2 display.
I'm just grasping at straws here, as Bob indicated it could be new encryption compatibility/communication standards.