Hopefully this is the same HAT, or at least (functionally) the same as the GeeekPi 4 channel relay hat - it looks the same...
... and the link below also references the GeeekPi Github repository.
From EP-0099
Register Map
Register Address Function Value 0x01 Relay 1 0(FULL OFF) - 1/255(ON) 0x02 Relay 2 0(FULL OFF) - 1/255(ON) 0x03 Relay 3 0(FULL OFF) - 1/255(ON) 0x04 Relay 4 0(FULL OFF) - 1/255(ON)
Note that this is relative to the base address, which is set by jumpers:
Base address
DIP switch status icon Device Address 0x10 0x11 0x12 0x13
The hat uses I2C, and so, from I2C - Inter Integrated Circuit, it uses pins 3 (GPIO2) and 5 (GPIO3) of the GPIO:
In addition, there is a repo with some demo code: dockerpi. In the 4channelRelay directory, the product link, on wiki.52pi.com, is broken. However, after some precise googling, the linked to product page would appear to be the EP-0099 link posted above.