I used to have a RPi 4 as a Jellyfin Media Server. I never tried 3-4 simultaneous streaming, but I remember that, if no server transcoding was involved, it could deal with 2 simulteaneous streamings without problems. I used an external HDD also.

On tutorials, Jellyfin's webpage has a nice tutorial on the Debian installer page (https://jellyfin.org/docs/general/installation/linux). Also, the container version could be a good option.

Watch out for folder permissions so Jellyfin can access your external HDD.

Hollow Knight and Vampire Survivors are the ones that managed got me hooked in the last few months