I went into this article with so much skepticism, but yeah, the author addresses all of my concerns. This is a cool project and now I get why you'd do it this way.
Do not buy a multi-charger for the RPi4, they may appear
to work, however if you look into things you will find
that they are "browning out". Official adapters are not
expensive, my advise is not to be stingy.
Probably don't want to get a new PoE switch, as I have a decent (fanless, silent) 16 port HP Procurve switch already that's barely utilised.
PoE switches never seem to be silent, and would annoy me. ;)
The other option I've been thinking of, is getting one of those "smart" power control boards + flashing it with Tasmota (https://tasmota.github.io) firmware.
That should work, and allow for using the official RPI4 adapters. :)