I don't have NROL4W yet but do have NROL. If the workouts are designed in an A/B fashion and 4 sessions of each is prescribed - then you set up your workout pattern of days and alternate A and B until you have done each 4 times (8 sessions total).
While 3x per week might be optimal and an easy schedule, not everyone can do that all of the time and so the books tell you how many sessions in total to do and let you spread that out over how often you can workout.
If you weight train 4 days a week, this will take 2 weeks to do A four times and B four times; if you train 2 days a week this will take 4 weeks; if you train 3 times a week it will take 2 and 2/3 weeks. Take that last Friday off
It may be that for me, my goal is 3x a week but in reality it goes like:
MWF WSa TuTh Su before I get 4 sessions of each completed. The point is, you don't do the program for a set number of weeks, you do it until you complete the prescribed number of sessions. But don't over think it.