No, unfortunately not. As soon as something has to be transcoded, the cpu goes to 100% and it stalls.

The container (MP4, MKV, etc) has nothing to do with it. Whether it judders or not depends on the video codec used and the end device (Plex client). I "only" have a DS220+, but Plex plays 4K HDR titles over the internet on an Android TV (e.g. Star Wars) without any problems, 100 Mbps is sufficient, so gigabit is by no means necessary. CPU utilisation is then 2-5% with Direct Play, meaning the thing is bored to death.
But if the end device can't handle a stream natively (i.e. doesn't understand H264/H265), it transcodes. At 4K, every Synology NAS goes to its knees. However, Plex kindly shows what is happening and what the reason is, simply press Pause, go to "...", select "Playback Info". The aim is so-called "Direct Play", then even an older Synology is sufficient. Otherwise, tools like Handbrake can help with the conversion.

I don't have 4k hdr movies, normal blue ray movies work fine.

doa melde mich später mal.

W

The problem is with .mkv files btw.
Some go others have the issue that it suddenly has to reload. Mp4 should have less problems