There are 5 workouts in the plan which get harder every week because the sets and reps (or time) increase.
When you start the workout plan, you're going to use a weight that's challenging, but lets you complete every rep of each exercise with perfect form. For example, choose a weight you can do let's say 10 reps for an exercise that calls for 8 reps. You're going to keep using that same weight throughout the program, so each week you're challenged to complete more reps. By the last week, you will lift that same weight for 15 reps. It should be hard, so if you're not feeling challenged - you should select a heavier weight.
The idea is that you cannot do the workouts in Week 4 right now based on your current strength levels and the weights you start with in Week 1. Over the course of 4 Weeks, you will get stronger and in better shape so that you can complete all the reps the final week.
This is called "progression" and is one of the reasons why this program works so well.