I’m assuming that neither pin (3 or 4) is grounded when the button is not pressed. Is this correct?
The fact that pin 3 does not ground when the button is pressed tells me that the foot switch isn’t connected properly. The solder joint is probably bad where the wire connects to the circuit board. One of the wires appears to not be making connection to the board…Or/and I suppose it could mean that pin 3 of the IC isn’t connected properly to the board….check the microcontroller’s solder joints.
You may need to watch some youtube tutorials on soldering and desoldering if you don’t have much experience with this.
Good luck.
-Brach