Roku Supports HDR10 over HDMI 2.0a with the Roku Ultra
Roku has been busy this year revamping their entire product line and choosing names for all their new products. Gone are the numeric Roku 4 and Roku 3 naming conventions. Whereas the Roku Express represents the lowest price point and only 1080p support, on the other end of the spectrum is the Roku Ultra (Click here for pricing on Amazon).
The Roku 4 was the first to Roku player to support 4K and when it was launched, you could only buy it directly from Roku. That has really changed with the Roku Ultra which can be ordered through both Amazon and Best Buy.
More significantly, the Roku Ultra also supports HDR10, one of the three available HDR technologies out there. Interestingly, the Chromecast Ultra has decided to support HDR10 and Dolby Vision while the Xiaomi 4K Mi Box (2016) supports HDR10 and HLG. Given HDR10's popularity in TV's, you should be able to connect the Roku Ultra to a lot of HDR capable 4K TV's out there. You'll need to be able to identify which online 4K content is HDR10 though to be able to get the proper HDR experience.
The only other Roku model that supports HDR is the Roku Premiere+ but go for the Roku Ultra if you want an advanced remote control similar to the one found in the Roku 4 which allows for voice search, remote audio headphone connection, etc.