Just been looking thru a lot of official Raspberry Pi 4 specs… usually a 3 Amp PSU is used, typical current draw on bare board is 600mA (0.6 a), with up to 1.2 Amp available for external peripherals… specific mention is made of (a) spinning hard drive causing problems, some modems, some higher power rated SSD’s… or anything that can suck power out of the system, in excess. The recommendation being use a good quality USB Hub for your external devices. If you use a crappy USB hub, that can back feed voltage into the USB port…bypassing the Pi protection circuitry (and indicator LEDs)…and you won’t know that the Pi is getting cooked. If excess current is being drawn, this loads the PSU and takes the voltage down. The Pi will still operate at this lower voltage… but it is in a ‘brown out’ situation and can cook the Pi circuit board
(from… Raspberry Pi Documentation - Raspberry Pi Hardware )
This doesn’t address POE which also uses the Pi PSU…so if you have POE plus any of the above problems, the POE current comes out of the total external current you may supply, and may crash and /or burn your Pi.
Also be aware that SSD current draw varies a lot between types…which is why the SATA drives are recommended as they use lower current than PCles and NVMe’s. Even the Argon M.2 base for the Argon One case specifies 1x SATA SSD only, and even then a PSU upgrade maybe in order… (a higher current model of PSU)
So the answer may be that to power up a Raspberry Pi 4 and 4 HDD’s off the one Raspberry PSU is simply too much for it to handle. I would definately do the powered USB Hub thing , after reading the cautions in the Pi documentation. BUT also refer to Powered USB HUB issues in the documentation… a whole extra level of complications and warnings about what you can and cannot do.
Definately read and understand your PSU and peripherals, and USB Hub recommendations within the Raspberry documentation.
Hope that this helps anyone out there…