Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Disconnect X4 from the PCB

Measurement:

Test 1:
Measure the resistance between the white and brown wires at the disconnected plug.
Resistance should be approximately 45 Ohms.
Test 2:
Measure the resistance between the black and blue wires at the disconnected plug.
Resistance should be approximately 125 Ohms.
Test 3:
Measure the resistance from each of the 4 wires to a suitable earth point.
Resistance should be greater than 1M Ohm on all 4 wires.