I don't have any experience with the rp4, but I have used a similar SBC (the Odroid N2) for this. An external HDD works perfectly fine, no need to worry about that. Performance is also absolutely decent. If the 4 devices can direct stream the content you won't have any issues. The only thing to worry about is transcoding. Ffmpeg on RP4 does support hardware acceleration (without it, you couldn't even do a single transcode at reasonable speeds), but even with it I doubt you can do more than a single 1080p transcode at the same time.
this post was submitted on 06 Jun 2023
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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.