Please go through the below troubleshooting steps.
1. Make sure the output resolution is supported by the projector
2. Make sure to use an HDMI 2.0/2.1 cable. Try testing with a different cable.
3. Make sure the OSD settings are correct.
- HDMI EDID set to Enhanced
- 4K Upscaling enabled to upscale all inputs to 4K resolution
4. Please connect the source with the projector directly, and bypass other equipment such as AVR
5. To support 4K streaming, make sure wired or Wi-Fi bandwidth is above 25Mbps
If the projector still cannot display 4K, please try to connect the source with other 4K displays or connect the projector to other 4K sources to check if the problem persists. If other 4K displays work, please contact the BenQ service center for further assistance.