RealTek 1619DR media players like Zidoo & Dune can downscale 4K video on the fly to 1080p while passing through the audio stream raw & untouched. It can even apply Dolby Vision RPU with its LLDV trick before the downscale to 1080p. Many Zidoo users with 1080p projectors have been playing 4K remux & ISO files. Zidoo has its own Home Theater software but has also released a third-party Plex client. Someone on the Zidoo forum has also written a "shim" to get the official Plex client working on Zidoo boxes.

I suggest that you check out Zidoo 1619DR product models. I have Plex running at home but prefer to use the Zidoo nowadays. Both my Plex server and Zidoo point to the same media files on my NAS.