Actually, after the information you gave, I expected you to ask for the voltage
across the resistor, so I'm not quite sure what you mean by 'PD'. It could be the
"Potential Difference" or the "Power Dissipation" across the resistor.
Here are both answers:
Power Dissipation:
Power = Energy/time = 400 joules / 4 seconds = 100 joules/second = 100 watts.
Potential Difference (voltage):
Power dissipated by a resistor = (voltage)² / Resistance
100 watts = (voltage)² / 4 ohms
Multiply each side by 4 ohms:
400 watts-ohms = (voltage)²
Voltage = square root of 400 watts-ohms = 20 volts
