I only changed the one that was open circuit. The others were around 1 ohm. It was so easy to do I thought there was no point in changing all 4.
The meter I was using didn't have any croc clips so getting a good contact wasn't easy. Touch the 2 probes together and ensure you get 0.00 before you begin, this will check the meter's battery. Touch one probe to the top of the plug and the other to any earth point (-ve of battery if you can reach).
Hoping I'm not teaching granny to suck eggs here.