For every 4 lists, a 5th list (bonus list) is free of charge. When you have 4 active lists, a link will appear at the top of your Account page where you can add the bonus list.
The bonus list's subscriber limit is equal to that of your largest list. As long 4 lists remain active the bonus list will never need renewing. If there are less than 4 active lists, the bonus list will also expire and will need "renewing" to keep it functioning.