To help with perspective, I think the tiny PCs we have in mind are like these. There won't be enough space for even a single 3.5" drive.
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For 4 3.5" HDDs, you might struggle to find even a Mini-ITX enclosure to house all that. More likely than not, you'll end up with a mini-ATX instead. Many mini-ITX PCs have one or even 2 M2 SSDs onboard, and some enclosures have space for a single additional 2.5" maybe 3.5" large capacity storage drive. Since they are smaller, the power supplies are also correspondingly smaller, though some mini-ITX enclosures can take full sized PSUs.
For the most part, many are also low-power devices, running off an external 65W or so power brick rather than a 200-300W mini power supply. So they are either using mobile processors or lower-end processors. The Pentium and Athlon names were reused by Intel/AMD to designate these lower end processors from their higher end Core/Ryzen product lines. They are still pretty capable machines for day to day use, or for running mini home servers, and more powerful than people realise. We tend to have overpowered PCs today, mainly driven by the gaming industry. But for most desktop work, a 4th-6th gen Intel processor is still perfectly capable - even enough for some photo editing.