I have a Netstream 4C and not the satellite version.
I borrowed another Netstream 4C, the situation is absolutely the same with it, no HD even the HD channels shown as very poorly resized, drop filed deinterlaced SD channels. While both Netstream 4C work with my iPhone in HD and my old Mac Mini with Eyetv3 in HD s well.
So I am pretty sure the problem is not the receiver hardware, but a kind of bug or some special setup issue.
I have a Hybrid T2/C USB tuner as well. It works in HD with the same Eyetv 4 license, but stuttering like hell, it is like 5-10 frames per second instead of 25. The deinterlacing options do no make any difference. I am using an 18 core iMac Pro with a 16 GB video card option and 64 GB RAM, so plenty of horsepowers to do proper decoding and deinterlacing.
I contacted Geniatech through their support, their Facebook page, their Twitter channel, and even on Instagram publicly and in messages.
No luck. No answer. No help. It is a shame.
I honestly can say they are far the worst company ever developed software for Apple operating system according to my almost 30 years of experience with Apple products.
I am dealing with Apple computers and all sorts of audio, video production, editing, and post-production systems since 1992.
Would you be so kind to let me know what Digital Device product you use with your Eyetv 4? Octopus NET? I feel the only solution will be to give up on either Eyetv4 or Netsream 4C or both. The Octopus Net seems a serious product if it can work on Mac (Big Sur) reliably.