First, you should determine whether it is really the lack of transcoding power which makes your videos “blocky”.
Install Tautulli and keep it open on a second screen, to see whether the playback is “Direct Play”, “Direct Stream” or “Transcode” while playing such a movie.
Some/most TVs don’t come with faster Ethernet ports than 100mbps. But 4K movies from UHD Bluray are exceeding these 100mbps temporarily.
So you have the absurd situation that using WiFi will work better, because the TV supports higher bandwidths over WiFi.
But you can only make use of this if your router is very close to the TV and supports these higher speeds over its in-built antennas.
Whether the movie you are trying to watch is exceeding the 100 mbps of the Ethernet connection, you can see in the movie’s Plex XML info
(No, looking at the averaged bandwidth of the file with other tools is nearly useless, because video files almost always are encoded with variable bitrate nowadays. So the ‘average’ numbers won’t show you the height of the peaks. )
When you have opened it, look for the first property called requiredBandwidths="11936,5545,4584,4584,4584,4584,4584,4584"
(my numbers are just an example)
The numbers you will see are the real required bandwidths in kilobit per second.
The first number is the worst case, if your player has only a very small network receive buffer memory. With Smart TV’s, you are usually in this category of player.
See also Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ
