That synology unit will not cut the mustard for 4k transcode at all.
Very roughly speaking, for a single full-transcode of a video, the following PassMark score requirements are a good guideline for the following average source file:
4K HDR (50Mbps, 10-bit HEVC) file: 17000 PassMark score (being transcoded to 10Mbps 1080p)
4K SDR (40Mbps, 8-bit HEVC) file: 12000 PassMark score (being transcoded to 10Mbps 1080p)
1080p (10Mbps, H.264) file: 2000 PassMark score
720p (4Mbps, H.264) file: 1500 PassMark score
I suggest you take some time to part out a standalone DIY build on whatever NAS OS flavour takes your fancy TrueNAS/unRAID.
Silverstone produce some very good NAS ordinated cases with backplanes making for easy install and swapping of drives, AsROCK Rack and Supermicro do great work in terms of server boards on the likes of AM4 and Intel.