It looks like some of the speculation about HDR (High Dynamic Range) capabilities has now been confirmed. Roku says via their support page that the following models will have HDR:
-Roku Premiere (Model 4620)
-Roku Premiere+ (Model 4630)
-Roku Ultra (Model 4640)
If you aren’t up to date on current TV standards, you may be asking what is HDR? Roku explains it best, “(HDR) improves picture quality by expanding the range of both color and contrast of any image. In more simple terms, in any scene, details that are typically lost in the brightest and darkest areas in a scene will have more details.
Wide Color Gamut goes further to expand the number of shades for each color to provide richer color depth, resulting in colors that are more true-to-life.”
You will need a 4K UHD television with HDR (and generally at least 25mbs download speeds depending on channel) to enjoy the Roku HDR support. But even without a 4K UHD TV your device will work in the standard HDMI port.