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.
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.