Big but old server, will it software transcode 4K?

[TheDreadPirate]

For a typical jellyfin user, the amount of writes to the SSD for transcodes is not a concern. You will upgrade before SSD endurance becomes an issue. Writing the transcodes to a hard drive might cause performance issues since a very capable GPU can transcode very quickly. Especially once you introduce random I/O into the workload.

RAM disk is an option. I do that myself (static RAM disk via fstab instead of /dev/shm) because my old ass SSD has pretty weak write performance. You just have to be mindful of how much space your transcodes typically use. My 4K content is relatively low bit rate and my RAM disk can support enough transcodes for my user count. If you have pretty much raw 4K HDR content, like 50-70Mbps and 50-75GB, the transcodes can be just as big. Maybe bigger.

Again, SSD endurance is really not a concern. But if you decide to try out a RAM disk, stress test it and monitor the RAM disk usage.