Hey jxsilicon9, your desire to stream video to multiple devices is definitely achievable if you only want to do 1080p. When you bring 4K into the mix, this opens up a whole world of hurt. Simply put, as long as your target devices can natively handle both the audio and video codecs of the source file, then you should be good. As soon as you bring transcoding into the picture, this is where things go haywire.
Something not many people seem to be aware of is that when you serve up source to a target and not want any transcoding, it has to be compatible BOTH from a video and audio codec perspective. If it is compatible from a video perspective, but not from an audio perspective, then both video and audio codec will be transcoded - at least I believe this is the case from a Plex perspective.
Although I'm not using a dedicated NAS, I was sort of hoping to do what you want to do via a dedicated NAS and fell into a whole world of hurt when I tried to move from 1080p to 4K. I've been using Plex perfectly for years now and it has been fantastic, but when I tried to make the move to 4K after getting a Samsung Q7F, it's been horrendous... My PC's just reached 5 years of age but is still in the top 76% of PCs running today when looking at it's overall performance. It's running an I7 Gen 3 @ 3.5GHz, 32GB DDR3 1600MHz RAM, 24TB of storage in raid 6 off an Adaptec ASAR7805 raid card that runs at 6Gbs. So to make things simple, it is one thing to transcode (what a Plex server has to do when the source file is not entirely compatible with the target device) in 1080p, and a separate thing altogether to transcode at 4K. Hell, I can't even play all 4K files in VLC or via the Windows 10 'Film & TV' app. Severe jutter and lag are what I generally get. To try and assist me here, I purchased a GTX 1080 Ti 11GB Xtreme GPU this weekend, and although it has helped quite a bit playing files directly from PC, it's still nowhere near perfect. With Plex Server, you can tell the server to offload as much processing as it can to your GPU's hardware - which also helped a bit with the new GTX 1080 Ti, but again, still not good. To give you an idea how much grunt you need to transcode on the fly, have a look at this post - https
/forums.overclockers.co.uk/threads/cpu-for-plex-media-server-4k-transcoding.18789345/.
So with respect to a NAS serving up 4K media (H264 - or even H265 where you need double the requirements for H264), I don't believe any exist today. If you open the NAS Compatibility Guide link from this page, you will find that 1080p is the best you will get - https
/support.plex.tv/articles/201373803-nas-compatibility-list/
If you need to know anything else, drop me a line...