I have a Pi 4 device, and cannot see the Keep screen on via HDMI option? i have tried this on both stable and testing releases.
Correct. This feature is not available on the Pi4 at the moment.
Could this be implemented?
Not sure. The reason this isn’t available yet is that CEC (the way the Pi interacts with the connected screen) isn’t fully available for both HDMI outputs on the Pi4. At least to my knowledge. This might have changed. I can check this.