[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.