4-LETTER PASSWORDS
1st letter choice, 26 possibilities; 2nd letter choice, 25 poss; 3rd choice, 24; 4th choice, 23.
26×25×24×23=358,800 total possible passwords. Not enough to cover 500,000.
6-DIGIT PASSCODES
Passcodes start at 000000 and end at 999999, a total of 1,000,000 different passcodes. Easily enough to cover 500,000 employees.
Therefore the 6-digit passcodes cover 500,000 employees and should be used. Having a surplus would allow for employees to leave while others join the company, using unique passcodes, until all existing employees have left and new ones replace them (one complete turnover of staff). After this, the password/passcode system would need to be revised.