Thanks for the quick reply gents.
I have checked my soldering, looks ok and I also inspected the nano board. looks new in fact. (I have decased it and shrink wrapped)
1. Initially I thought servo wire #4 could be an issue but it powers motor #3 fine.
2. I also swapped motors but same result.
3. So for some reason pin #4 is not sending signal to spin the motors. In fact, during the GCS motor calibration stage, I notice it could not find the min throttle point easily. (the other 3 motors spun smoothly at 1020 or so but motor 4 did not spin till say a much higher value, can't remember the value but the slider was greater than mid point)
Are you suggesting it is a software problem, assuming the pin 4 structurally looks fine?