4x 4k transcode with high bitrate simultanously is a big stretch for small systems like this. Transcoding HDR Tonemapping is demanding and keep in mind that not only video has to be transcoded but often audio as well which can be too much in terms of CPU performance.
For a powerfull transcoding NAS I would either go Asrock Deskmeet B660 with an Intel Arc A380 or build something out of an Asrock Deskmini B660/B760 with an i3-12100/13100 or i5-12400. They can hold 2x m.2 and 2-3x SATA drives for a reasonable price.