So connecting the reference pin between the zener diode and the resistance and measuring the voltage is the calibration procedure?
Just connect the four single cells to four analogue inputs, and get the A/D value of each input with the internal reference enabled in void setup(). That should give you A/D values of about 500-600.
A maths line in your code can convert that A/D value to voltage.
The maths line should contain the reference voltage of the Arduino you're working with.
Two ways to calibrate.
1) Measure voltage on the Aref pin with a DMM, and use that voltage in the maths line.
2) Measure e.g. voltage on first solar cell under direct sunlight with a DMM, and see if the serial monitor displays the same voltage.
Change the Aref value in the maths line, and re-upload, untill the serial monitor displays the same as the DMM.
The second approach could be more precise.
Write the Aref voltage on the board for future reference.