I'm using the ThumbDV without issue. The solution for me, not skipping any of these steps:
1_ Make sure that the Pi4 works with DMR and the default audio driver. If not, ThumbDV isn't the issue.
2_ I picked up a "Nekteck 72W 4 port USB C wall charger station" from Amazon.
3_ Plugged my entire Raspberry Pi4 into it the Nekteck. I have two Pi4's running off of it without issue, one with ThumbDV one without. That's all that I have powered.
4_ Plugged ThumbDV in one of the Pi4's USB 2.0 ports. Run AMBEtest4 a couple of times to be sure it's working.
5_ Run AMBEtest4 a couple of times and verify that the ThumbDV passes.