The rules for AF4 can be found by clicking here.
This is a straightforward system for processing an undifferentiated list. The list is divided into two parts, the Closed List and the Open List. Each time all the tasks in the Closed List are finished or dismissed, the existing Open List has a line drawn at its end and becomes the Closed List. Tasks added after that are the new Open List. To process the list, first you circulate around the Closed List until no more tasks are ready to be done, then you are allowed one pass through the Open List before returning to the Closed List.
Because of the nature of the two lists, the system allows urgent tasks to be dealt with while still encouraging the more difficult tasks to be done.
When the Closed List is down to a few items, the vast majority of tasks are in the Open List. This means that there is often a considerable time lag before any pressure is applied to complete a task that is in the Open List. One result of this is that the little and often principle doesn’t work well with the more difficult tasks.
AF4 is probably the most generally effective of the four Autofocus systems. In spite of its drawbacks many people have reported that it has continued to be effective after a long period of use.