Re: Roku Ultra specs say it supports EAC3 codec - not for me
You didnt mention your model #/firmware version, specific method of playback (USB/UPnP/etc) or app (RMP/Plex/EmbyJellyfin/etc) or any specific file type/encoding tool/process, or how your 4800 is connected, so there may be an issue with any of those aspects.
The 4800 does support DD+ (EAC3) from both streaming services and local file (USB/UPnP/etc) playback (every file with EAC3 support I've thrown at it using USB/UPnP via RMP plays fine).
(Additional unlisted file types/formats may be supported depending on model/firmware version)
There are some issues with EAC3 + HEVC/x265 encoded files, but they result in color/contrast distortions, not lack of audio output.
1) Make sure you use Passthrough: Settings/Audio/Digital output format = Passthrough (if you leave it on Auto all audio output will be transcoded/converted to the highest detected Dolby level of the connected device)
2) Make sure DD+/DD support is detected: HighlightSettings/Audio/Passthrough and check the "Available:" formats (In Auto/Passthough/Custom w/DD-only detected/configured, DD+ will be converted to DD upon output)
3) Try a different file/format/encoding tool (and/or post a link to a sample file of yours with EAC3 that has no playback for you)