Netflix should play HDR content, but not 4K, and is only available in mostly Netflix Originals (and not all of them). I'm not certain, however, if you can get HDR content in Netflix if you have the base plan or need the higher tier that includes 4K streams. Vudu has not been updated to support HDR on those devices (and not sure they ever will, don't know their policies on mobile). You should also be able to play HDR content in YouTube.
That being said, your phone is NOT 4K (S8 panel: 2960x1440 4K: 3840x2160). Make sure that you didn't set your phone's resolution to anything less than the full resolution as this may bork apps' HDR detection.