I'd love to find an adapter for this so I could plug a generic 4-pin PWM fan without having to rewire everything (which I don't trust myself to do properly).

The board generates PWM signals from its GPIO pins, which are then sent to an AC Fan Speed Controller module that modulates the fan speed accordingly. At the same time, standard relay modules are used ...

Something to remember about the entire Pi family is the pins on the Broadcom processors are multipurpose. Does it increase the confusion or the capabilities? Take your pick. But the key benefit ...