Shouldn’t need to transcode here. The CCWGTV should play the 4K content natively and then downscale to 1080p for sending to the display.
If content is actually cropped like you said this sounds like a playback settings issue. Check the Plex app to make sure it’s set to Letterbox mode (to add letterboxing to content with aspect higher than your TV). Also check the TV’s settings regarding zoom.
Install the Downloader app. Then you can browse the Plex for Android thread and download/install previous app versions of the Plex client app.