[noggin]

Does anyone have any advice on what to get if you need something that can downscale from 4K to 1080P, and also bitstream HD audio?  I have an old Dell Chromebox 3010 that can't do it, so wondering if anything out there can do this.  I'd prefer to stay with a Chromebox since I have had zero problems with LibreElec to date.  No real budget, just want it to work like the Chromebox has for many years 

Are your 4K sources HDR Rec 2020 (i.e. HDR10 - which is normal for UHD movies on Blu-ray) or SDR Rec 709 (which is how some online services can provide UHD content), and are they h.265/HEVC 10-bit (common for UHD Blu-ray) or h.264/AVC 8-bit (used for some online content)?  

Is your HD display Rec 709 SDR only, or does it support Rec 2020 HDR (as some newer displays do)?

Those variables decide what codec support you need, and whether you need to just downscale (2160p/4k->1080p) or whether you need to tonemap (convert HDR Rec 2020 to SDR Rec 709) as well.  Tonemapping Rec 2020 HDR to Rec 709 SDR is never going to be perfect, as you are losing a lot of information in colour and dynamic range terms in the conversion - and some people are not that happy with the results on some platforms.

Wow, thanks - that is super informative and I had no idea of the complexity involved.  Given our viewing habits I'm guessing I will need to deal with everything (i.e., HDR Rec 2020 and SDR Rec 709, h.265 and h.264).  My receiver and TV are both 1080P (Pioneer Elite SC-05 and Samsunf F8500) so I'm guessing these are basically all Rec 709 SDR, which would require the tonemapping you mention.

Taking all of this in the account, would a Chronebox with a 10th Gen Intel Processor (Ice Lake) be capable of this?  Otherwise, would I need to get something like this HDFury device ?

I'll leave others to comment on suitable boxes - but a key requirement you have is 4K UHD HDR->HD SDR conversion - so both replay AND tonemapping performance are requirements for you.

I can't comment on the HD Fury Diva's HDR->SDR conversion quality - there is no single 'right' way of doing that conversion - as there are a number of different approaches that can be taken, each with their own merits.