In the calibration.md file, you will find the following statement:
set voltage to nominal battery voltage (7.5V for 2S, 11.3V for 3S, 15V for 4S) make sure the power supply can handle the maximum motor current. If not, you will see the voltage drop towards the end of the test (power supply limiting) and the calibration will be invalid
You should be able to determine the maximum motor current based on the motor specifications provided by the manufacturer. For 4S, you would set the power supply to about 15V (would need 16.8V if you wanted to see absolute maximum RPM achievable with a 4S battery)
Using a power supply is a suggestion, but you could just use a battery as well. You can monitor the current drawn by the ESC (the calibration script should print it on screen) and abort the test if you see or hear anything unusual.
Alex