If you want 4K HDR Transcoding + Tonemapping, you must run Plex Media Server in Docker.
See Synology FAQ #28. Using Docker for HW trancode & Tone Mapping - How To
Hardware accelerated transcoding uses the Intel Quick-Sync GPU embedded in the J4125, not the CPU cores. GPU Utilization is not displayed via Synology Resource Monitor, top/htop (cli), etc.
Tonemapping using the GPU requires the Intel Compute Runtime libraries. The libraries are not installed by Synology and not bundled with Plex Media Server. They are part of the Docker package. The J4125 is not powerful enough to tonemap in software, using the CPU cores. Hence the need to use Docker.
