There are a few other options besides Trinnov, Arvus, StormAudio, Lyngdorf. In my opinion, these are priced incomprehensibly, low quantities already considered. So, if you can abstain from Atmos:
Theoretically, it should be possible to replace an AVR with a Raspberry Pi and an add-on board which features 4 AES/EBU outputs.
The crazy thing is: you can buy a Pi with 2 channel AES out and you can buy one with 8 channel driverless analog out
and HDMI input(!) which decodes (not extracts) up to Dolby TrueHD.
What you can, of course, not buy, is an add-on board with 4 AES outputs and only god knows why. It is insane.
Getting more practical in all digital:
Option 1)
As you only need 4.1: The VanityPro costs ~ 1500.
This works
only if you are able to get the decoding stage moved to the TV, BluRay, AppleTV (or whatever you use as source). All these devices are capable of outputting LPCM 8 channel uncompressed. The VanityPro will extract that and outputs this as AES3. Regarding this, you might find more info from myself and others if you dig deep enough in this thread:
This is a review and measurements of the Audiopraise VanityPro HDMI audio extractor. It was kindly sent to me by their distributor, JVB Digital and costs US $1,599. The VanityPro is a compact device, not much bigger than typical desktop audio products. Its main functionality is to extract...
www.audiosciencereview.com
Option 2)
Remove HDCP via HDFury or similar trustable components. Convert HDMI to SDI. Convert SDI to multichannel AES3. Feed output to speakers. Needs also decoding in source; same restrictions as VanityPro apply here. Problem: It is a giant mess consisting of cables and boxes.
Option 3)
Get one of the older Dolby/ DTS decoders made for Film authoring: DTS CAD 4/5 or Dolby DP564 etc.
The good: decodes directly Dolby or DTS to AES via optical in.
The bad: decodes directly Dolby or DTS to AES via optical in. You are running an all digital network but compressed instead of lossless (as would be the case of an analog network).
Option 4)
The future, aka AES 67. Purely academic, because: requires a whole new setup. See
https://www.audiosciencereview.com/forum/index.php?threads/new-product-arvus-h1-d.47117/