I'm confused about the marketing on the new RPI 4. Here in the tech specs it says "Power over Ethernet (PoE) enabled (requires separate PoE HAT)". What does it mean it's enabled, but it needs a separate hat. The RPI 3 could be powered with POE using a hat, what makes this any different?
It means the hardware with a POE HAT will work without any special software changes needed. That means the support is built-in to Raspbian Buster (which was launched today with the RPi4).
It takes zero software to accomplish POE on the RPI, only breaking out the wiring from the ethernet connector to a 4 pin header, which is done for you already on RPI 3B+ and 4B, sending it through the DC-DC converter of the Hat and then powering the PI through the GPIO connector pins.
On the POE Hat, there is a Fan, this fan will turn on at a certain temperature threshold. This requires software, but most people use these fans without a POE hat, so it's really not because of the POE hat that there is software for the fan.
The RPI 4B power specs are 5.1v at 3.0amp. The PoE hat output matches the RPI 3B+ requirements of 5V at 2.5Amps. The PoE hat can and is being successfully with RPI 4B. However, it behooves the user to insure that the draw on the RPI 4B is at the lower rating. Easy on peripherals and such.
The RPI power specs can be seen at https://www.raspberrypi.org/products/raspberry-pi-4-model-b/specifications/. That page also notes that the older RPI power supplies can be used with some caution.
The PoE hat specs are in a pdf at https://static.raspberrypi.org/files/product-briefs/Raspberry-Pi-PoE_HAT-Product-Brief.pdf
For earlier models of Pi, up to and including the 3B, there existed third party solutions for PoE, but all of them required the network cable to be looped through the PoE module. This lead to rather bulky soloutions, often effectively doubling the height of the Pi.
On the 3B+ and 4B, the Ethernet jack was replaced with one that supported PoE and the cable side center taps were routed out to a new 4-pin header. This allows a PoE hat to be implemented without needing to loop the cable through the hat and hence allows for a much neater and lower profile solution.
You will find the line about PoE in the specifications for the 3B+ and the 4B, but not for earlier models.