I would agree that I found NBME 5 to be much easier that 4, but many people say that 3 and 4 are way tougher than the actual thing. I think it gives you a guide to see that you've made some progress, and I made huge gains as well between 4 and 5, but I think I'll still take another 2 exams to make sure my scores are still improving and not going backwards. I'll leave 3 for the last one as I think that is a more difficult one, and will help me towards the final push...only 4 weeks left until d-day!