As @OttoKerner mentions, you’ll need a 7th or 8th gen Intel CPU to get support for 4K HEVC hardware acceleration.
Your current GPUs do not support HEVC. Capabilities are shown in Nvidia’s Video Encode and Decode GPU Support Matrix.
Review Using Hardware-Accelerated Streaming for additional information.
You do have some options other than a new CPU, which also means a new motherboard & memory.
You could get a GTX 1050 or better GPU (see aforementioned matrix). Note that Nvidia GTX GPUs are limited, by Nvidia, to two concurrent transcodes. The third concurrent transcode will use the system CPU, not the built-in Intel GPU.
You could also go with a Quadro P2000 or greater card. See this post: Quadro P2000 + old $75 desktop capable of 21 h265 transcodes
Addition:
One other option, which doesn’t cost anything, is to simply restrict 4K material to 4K capable devices.
I keep my 4K material in separate libraries and do not make it available to my remote users (none of which have 4K systems). This eliminates the need for some serious CPU cycles to transcode 4K movies. I can run PMS on my NAS w/ a Celeron CPU.
Also, transcoded 4K HDR doesn’t look very good anyway. Plex is still working on tonemapping 10-bit HEVC to 8-bit H264. As such, colors on the transcoded video look washed out. It will get fixed eventually. However, for the time being it is best to avoid transcoding HDR material.
