Check out the mini pcs by Minisforum and Beelink

I use the Beelink MINI-S12 and it’s worked really well. Its been able to handle 4k streams without any issues.

Whatever you get, it can be helpful (and recommended for less experienced users) to go for a box with an intel CPU that has an iGPU to allow for QuickSync hardware transcoding. Plex doesn’t work with AMD AMF, and I’m assuming you won’t have a dedicated GPU, so QuickSync will be a must if you want buttery smooth transcoding.

I'd look into something with at least a Intel® UHD Graphics 730 iGPU.

It'll give you some flexibility to add hardware transcoding in the future.

Honestly if it's direct 4k stream I feel like any modern mini PC will do, especially NUCs. Your bottleneck will be your network. I'm having trouble playing direct over wifi 6, so I recommend hard-wired if possible.

I would recommend anything Gen 8 Intel or higher. The quick sync is just so good.

If you can tolerate a slightly larger form factor than a NUC, you can often find better deals on SFF PCs on ebay or from refurbished office equipment sellers. For example, a small Dell Optiplex or a HP or Lenovo would easily meet your spec requirements, plus they have space for a hard drive inisde.

Thanks for all the advice and that really good to know about amd as I almost went that way. I was looking at the beelink before posting and I think I’m going to grab this as it’s on $200 right now.