How necessary is extra cooling for the RPi4 now?

[Imperf3kt]

The only thing that troubles me about the official PoE HAT is it is rated at 2.5A which is fine for a 3B but the Foundation themselves recommend a 3A PSU for the Model 4 despite their own PoE HAT being listed as compatible?

It is compatible with a small side note that you lose about 0.5A toward USB and other peripherals.
If you aren't using more than 0.7A worth of Usb devices, it will perform identically. If you are, the Pi rarely uses maximum power anyway unless you're really pushing it.

[davidcoton]

The only thing that troubles me about the official PoE HAT is it is rated at 2.5A which is fine for a 3B but the Foundation themselves recommend a 3A PSU for the Model 4 despite their own PoE HAT being listed as compatible?

It is compatible with a small side note that you lose about 0.5A toward USB and other peripherals.
If you aren't using more than 0.7A worth of Usb devices, it will perform identically. If you are, the Pi rarely uses maximum power anyway unless you're really pushing it.

It's not even that bad. A stressed Pi4B draws 1A25, USB peripherals are limited to 1A2. So a Pi POE HAT is adequate unless you also have some load connected to the GPIO. Alternatively, at a lower USB load you can still have GPIO peripherals -- it's a direct trade off.

If you are drawing 1A2 through GPIO and USB combined and pushing the Pi hard, you are on the limit for a 2A5 supply; some may be dropping their voltage at full load. I haven't seen a load graph for the POE HAT, though.