1. It’s hard to know for sure about your needs without knowing the cameras you’re using, the compression, bitrate and color spec at which you want to record, etc.
2. First of all, I’m not sure that it’s so simple to record from all four of the Decklink Quad’s inputs simultaneously. At the very least, you need a very powerful machine... encoding four videos simultaneously would be very taxing on a system, if it’s even possible. The Decklink Quad is great if you want to use software switching (like Wirecast or vMix or OBS) to broadcast live. But if you want to record four cameras, most video creators record them each separately. So if you don’t need live multi-camera video, and in-camera recording (to whatever media your cameras use) is for some reason not an option, I’d use recorders like those made by Atomos or Blackmagic. (I’ve had a lot of success with the Blackmagic Video Assist line.) I suppose you’ll need four of them, but that means not needing the Quad capture card, and not needing quite as high-power a machine (because you won’t have to ingest 4x 4K video streams at once). When it’s all done, you’ll have to ingest the files for post-production, but that’s how most video creators do it.
3. If you can afford big enough M.2 SSDs for your needs, don’t bother with HDDs in RAID. (If you need the space for archival video then fine, but for editing a single project made up of 120 minutes of standard bitrate video recording from four cameras, 2-4TB should be plenty.) Not using 4 or 5 HDDs mean you need less space in the case, less power, less cabling, and not having to deal with a RAID card.
4. For Premiere, you’ll get good performance from at least 64GB of RAM and an i7 or i9 CPU with as many cores as possible. In other words: as beefy as you can afford for your budget. And you’ll want the highest-end current-model NVIDIA card you can afford to put in there.