The board of directors in its discretion and under rules made by it may retire the unpledged shares of any class at any time after four years from the date of their issue, by enforcing withdrawals of the same. However, the shareholders whose shares are to be retired shall be determined by lot, and they shall be paid the full value of their shares, less all fines and proportionate part of unadjusted loss, and shares pledged for share loans shall be treated as unpledged shares.