Yeah. I've been wondering about this as well.
Is the 3A requirement for the RPi 4 because USB-C only offers 500/900mA, 1.5A and 3A or because it does actually want 500mA more than a 3B/3B+? The PoE HAT is listed as compatible with the RPi 4 but as mentioned in the above post, it only does 2.5A. Can it fully power a RPi 4 with everything installed/connected because the actual maximum curent draw is 2.5A or should we be conservative?
The PoE HAT would have to be upgraded to support 802.3at aka PoE+ if we want 3A out. 802.3af supplies 15.4W but only guarantees 12.95W delivered to the device due to cable losses. 12.95W would be 2.59A at 5V. Trying to draw more will cause most PoE switches to cut power entirely as it'll detect it as an overcurrent fault.