Amlogic s905x or s912x theoretically support 4K HDR, but not on LibreElec (see here, "4K support is experimental. 3D and HDR are not supported, i.e. they might work but I don't provide support."). I guess you could by any TV Box with these chipsets, just be careful with the WiFi chip and gigabit Ethernet (if important). My Tanix Tx7 works good (I have no HDR or 4K TV to test though).
According the the below thread the s905x and s912x do support HDR10 with LE though or am I wrong? Also I was still a little unsure about DTS-MA/True-HD audio support due to licensing.
Auto frame rate switching & dynamic range matching - 4K (HDR10) capable Hardware