There is just one more agreement, but it's the one that allows the other three to become deeply ingrained habits. The fourth agreement is about the action of the first three: ALWAYS DO YOUR BEST.
Under any circumstances, always do your best, no more and no less. But keep in mind that your best is not going to be the same from one moment to the next. When you wake up refreshed and energized in the morning, your best will be better than when you are tired at night. Your best will be different when you are healthy as opposed to sick, or sober as opposed to drunk.
Regardless of the quality of your efforts, keep doing your best. If you try to do more than your best, you spend more energy than is needed, and in the end your best will not be enough. When you overdo, you go against yourself, and it takes longer to accomplish your goal. But if you do less than your best, you subject yourself to frustrations, self-judgment, guilt and regrets.
If you have done your best and your inner judge tries to judge you according to your Book of Laws, you've got the answer: "I did my best." There are no regrets. That is why you always do your best.