Yes, the 4800/4802 supports AC-4 decoding to AC-3. I have no issues using my 4800 for ATSC 3.0 broadcasts. That said, Silicon Dust isn't putting much effort into the user interface for the Roku channel. It's far better on my Shield, but that device doesn't support AC-4 so they perform some sort of Internet magic to decode the audio online. It works fine, but I'd prefer the decoding performed locally, like the Ultra does.
I'm not sure what sound issues you are referring to. I haven't noticed any audio issues using either the Ultra or the Shield when viewing ATSC 3.0 content.