Once you complete workout 1, move on and do workout 2 the following week, then workout 3 the week after that.
Week 4, you take the exact same weight used in week 2, however this time you aim for 5 reps not 4.
Week 5, you take the exact same weight used in week 3, however this time you aim for 4 reps not 3.
Week 6, I suppose you again increase on week 5 by 6-7%.
So to basically answer your question, regardless whether you are hitting the target reps in your last few sets, you move onto the following weeks workout plan.
I'm actually considering this program having done the original GVT with great success a number of years ago. First I have to rehab my dislocated elbow injury though
Let me know how it goes, PM me when you complete it because I'll surely forget to PM you.