First, check whether there is continuity between the terminals of GND1 to GND4 on the board with the DMM.
If there is no continuity and the terminals from GND1 to GND4 are independent, try the following procedure.
Connect all PWMn terminals on the board to 5V (that is VCC of PCF8574), and connect GND1 to 4 to the output pins of PCF8574.
Then, the logic of the output signal is inverted.
@anon35827816
Please review the above process. 