Hi @fatalwario,
A quick update regarding your issue.
We recommend enabling "UHD Color" in your Samsung TV's HDMI settings. To do this, use your Samsung TV remote. Afterwards, please let us know if the issue persists.
Thanks,
The Roku Community Team
I finally found that setting, now 4K works.
Thank you very much for your help
Thank you for keeping us posted @fatalwario.
We are glad to hear that the troubleshooting steps we provided fixed your issue. Please don't hesitate to reach out again if you have more concerns about your Roku.