I'll check now, thought they'd gone non-rooted devices only to actually watch the video content. I remember there was a workaround involving the Dolphin browser and a flash APK from an xda developers forum thread. Mind you, not looked at it since they changed to the All4 from the old 4oD name. Double checking, it was definitely this one -
https://play.google.com/store/apps/details?id=com.channel4.ondemand&hl=en - that you installed from the Play Store?
edit: workaround didn't involve the old 4oD app, just to be clear, it was visiting the website version in a browser.
edit2:
Yes, because the device is rooted, they don't want to play nicely:
Back to Genesis and iStream in Kodi then, shame.
You could try asking them if they'll ever release a version that doesn't reject rooted devices, or if your 12 month warranty has expired or isn't important, you could look for a non-rooted firmware to flash from a 3rd party. Possible it will work, but I'd not take the risk on my own device without really needing non-rooted status.