How compute saving plan going to be adjusted?

I am very much confused in terms of how saving plan going to be adjusted between those instances? As an example, if we purchased 4 instances, each instance would cost $5 per hour and a saving plan would cost $4, so we purchased a compute saving plan by committing $13 per hour How this saving plan will be adjusted between those instances? Since 3 instances would consume $12 how $1 going to be adjusted? because we have purchased Rs.13 per hour saving plan?