Best answer: No, the DS419slim is not intended for 4K movie playback. If you need to transcode 4K movies to the same resolution or lower, you'll encounter buffering issues since the low-powered CPU and 512MB RAM attempt to keep up with the intense system workload. You'll need a more expensive NAS.
Not designed for Plex
The DS419slim is a niche NAS and isn't designed to handle a suite like Plex Media Server. For starters, you've only got 512MB of RAM. That's nowhere near enough to run Plex comfortably. Then you have the low-powered CPU, which is great for general file storage and some other tasks, but you won't get far when playing media, especially at 4K.
You won't be able to replace the Marvell Armada 385 88F6820 CPU, nor will you be able to upgrade the small amount of available system memory. The DS419slim just wasn't designed for 4K playback. Where this NAS really shines is in the portable NAS segment, allowing you to store all your files on solid-state drive (SSD) storage inside a chassis that weighs just 1.5 pounds (0.66kg)
You may get away with streaming some files without issue — if everything matches on the end of the Synology NAS and the recipient device — but it's worth spending a little more and buying an Intel-based NAS if you want a smoother experience. Even more pricey NAS options from manufacturers can struggle with 4K transcoding.
Prepare to spend serious money on 4K
As aforementioned, even pricey NAS solutions from Synology and other vendors may struggle with 4K content that needs to be transcoded. One way of dealing with this is to re-encode movies on a PC to a format that can be played on all your devices. This frees up resources on the NAS when you attempt to watch them, but if you need to transcode on the fly, the DS1019+ is a decent option to get started.
Dropping the 32-bit ARM processor for a quad-core 64-bit Intel Celeron processor with better support for transcoding, and swapping 512MB of RAM for 8GB makes a huge difference in not just Plex, but other apps and processes you may run on the NAS. It's not the perfect NAS for 4K (and beyond) playback, but it's significantly better than the DS419slim in this area.
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