It shouldn’t really scale above a certain point, it’s just that if you import a lot of pictures all at once (in my case with immich-go, as mentioned from a Google photos takeout), all the ML-Tasks (facial recognition, search tagging) together with the normal tasks (thumbnail extraction, video transcoding) can suddenly increase the ram usage from in my case normally 1-2gb to around 6, which even on an 8gb raspi could get a little tight. But for example even just 16gb in the HP should be enough, assuming you don’t have a bunch of other stuff clogging up your RAM. As for the CPU, I’m not too sure tbh, I’m using a Ryzen 5 5600G, which can handle everything that’s running on my server with ease. I think the only significant downside you’d have here going with 4th gen is power efficiency. I still have an i7 6850K (OCd to 4.3GHz) in my main PC, and that thing uses a couple hundred watts when fully loaded, compared to my Ryzens 65W or so. If you’re just going for an i5 or i3 with the fourth gen that’s probably not gonna matter too much, but the performance probably also won’t be amazing at the same time.

I’d also highly recommend the proper PC, Immich can get pretty RAM-hungry if you use the ML functions, for me that has actually caused crashes before. Granted, that was while importing roughly 20k Assets (200GB) from a Google photos takeout, but it’s still probably better to be prepared.