First let's look at a couple of things about 4K.
1) 4K gets it's name from being 4x the pixel count of 1080p HD video. This is due to the screen having the same "density" in 1/4 of it as a 1080p screen has across the whole thing.
2) 4K also uses 10-bit color depth where blacks are blacker and whites are whiter. The 1080p standard still uses (generally) 8-bit color depth.
So the information on the TV is 4x as much info due to pixel counts, as well as an additional 20% due to color. For instance, a 1080p version of a movie could be as high as 20Mbps bitrates. Where as the same movie in 4K is going to start at 45Mbps and might go as high as 65Mbps or 80Mbps. (We're not going to talk about 6-channel audio over 2-channel. that should be a no-brainer, but nothing to do with video.) This increase in pixel count can really become CPU intensive if anything needs to be transcoded for any reason. (And it is sometimes hard to find out the reasons for transcoding.) The receiver might be able to take the stream straight from the NAS, but if the TV can't, transcoding happens. Or the other way around.
Plex Media Server could handle this without transcoding, if there is a Plex client app for your TV, and if the app supports 4K on that model/make. That's going to be up to you to research. BUT, with transcoding, that NAS hasn't got enough CPU for consistent RT transcoding. (I know, that's the model I have as well.)
Right now, I'm not sure of any streaming apps that can reliably handle transcoded media when it's 4K. That's a lot of data to be pushing through the pipeline in a short period of time. (CPU, memory, disk reads/writes, and then network. Everything else has to happen before it hits the wire...) If the media doesn't require transcoding, you just have to worry about reads and the wire. (Not trying to run this wireless are you? If so, rethink it!)
Plex Media Server makes a recommendation of 2K passmarks for a 1080p 8Mbps movie if transcoding is involved. Just doing the simple math of 4X means the passmarks for transcoding is going to require 8K passmarks. That CPU has about 5K passmarks. So if there's no transcoding, the CPU could handle quite a few streams. If just one stream requires transcoding of a 4K stream, none of the people watching any stream from the box are going to be able to watch anything.
CPU technology has to come a long way before everyone will be watching their 4K streaming media from their own devices.
H264 is reliable and consistent on that box. H264 doesn't support 10-bit very well, though.. H265 is new, not all together reliable and problematic to get devices to work with this codec. Yes, it's smaller file size is appealing, and it can easily deal with a 10-bit video stream, but, most CPU's aren't able to work with it very well.
Last thought and I'm done for now. 4K seems nice. But most people couldn't tell you if they are watching a 4K stream or a 1080p stream from the realistic watching distances. (
https://carltonbale.com/does-4k-resolution-matter/ for reference) Is it worth spending a bunch of money to get something you may not even be able to see a significant difference with? Are you sitting within 6-8 feet of that 90" TV? If not, you likely couldn't tell the difference which stream is which. (And if you can, it's probably time to call Ripley's as you have better eyes than most of the rest of humanity.)