The maxage option , as you know, forces your users to change passwords after 4 weeks and indeed will ask them to change their password next time they log in. If you also set the maxexpired option to 4, then 4 weeks after the password expires the user will no longer be able to log in and change their password. When they attempt to log in, they will get a message telling them that their account is inaccessible and to get it reset by a system administrator.
If using this do consider that it will not lock an account that has not been used for 28 days, only an account that has not been used for 28 days since password expiry. If the person changed their password immediately before leaving it will be 8 weeks before the account locks.
The account expiry option won't help you, it's main use is to lock an account on a known date. e.g. a college might have an academic year of October to June and want to lock all student accounts after the year ends and so set an expiration date when creating the users.
Using a script like the one I suggested would explicitly lock accounts unused for 30 days and also it is v easy to get a list of locked accounts just by processing the /etc/password file and checking the account_locked flag from lsuser.
Dave