Try running this image Armbian on T4. It includes DTB for nanopc-t4.
Wait a Moment ... I will test it.
So, I tested it with the DTB directly from the image.
Boot, EMMC, Bluetooth -> ok
WiFi -> same problem with non-loadable firmware
Video -> ok
At first it wasn't possible via HDMI. I tried it via USB (changed via Default Sink), it worked. Changing to HDMI afterwards was ok.
After every reboot the same problem, switch manually once and then sound came over HDMI.
Under Debian there was no problem, only here under Ubuntu Bionic.
The second DTB file in your download link, is it the same (right size) as in the image or should I do a test with it?
I would still have time for a test today, then again at the weekend.