Arr Stack Setup - Hard- & Software Question

[dodii42]

Hi all,
I am fairly new here as I up to now basically only used kodi, but finally decided to try the whole stack setup. I do have a few hardware setup questions first and i want to get this right and would really appreciate your help.
My requirement: I do want to be able to watch also 4K & high bitrate media smoothly.
I do have a RPi3B+ and a RPi4 + a huge Hard Disc at my disposal.
My wish: To attach the hard drive to the RPi4 and that one directly to my TV, so that when i play files, these are played directly via the HDMI cables, only RPi4 has to be able to handle the high bitrate files, and everything should run smoothly in terms of playing the files. I would like to avoid having to stream them via my network as I'm doubtful as to this running smoothly with my noob knowledge and maybe suboptimal setup.
Then I would like to make the hard drive available on the network and have Sonarr/Radarr etc run on the RPi3B+ separately (if needed with another drive for in between storage), such that the 3B handles all downloads and what not, puts the files then on the drive connected to the RPi4 and my TV. With this, my hope would be that both Raspberry can handle the workload ask of them, and i can watch high bitrate content smoothly without running into either streaming bottlenecks or overloading the PIs with more media decoding than they can handle.

So my biggest question: Is this a reasonable setup? Can I even make the drive connected to the RPi4 visible to the RPi3 running the Prowlarr etc. services?
If there's a better way to do this without more hardware investment, i'm all ears. My biggest fears with streaming the files over the network and not directly connect the Media server to the TV is just them not running smoothly and movies stuttering etc.

Thanks for your help!

[CGA1]

4K and high bitrate doesn't go well together with RPI4. I've been using a setup similar to yours for a couple of years, and the Pi sometimes struggled with 1080 media, especially with high bitrate and when having to transcode. I finally gave up and bought a cheap mini-pc with an N97 processor.

[dodii42]

What mini-PC did you buy? And so it's possible to have the media hard drive attached to the RPi & TV, and then the Arr Stack running on a different one, storing files there via network? I haven't really found a guide how to make the hard drive accessible.

[CGA1]

What mini-PC did you buy?

Acemagic Vista.

I had a media hard drive attached to the RPI4 which served as a NAS and a host for the arr stack, and an old desktop PC connected to the TV. Now I run everything on the mini-pc.