Sforno on Exodus 4:21/ספורנו על שמות ד׳:כ״א

1בלכתך לשוב מצרימה, every time I tell you to leave your tent which is outside the boundary of the Egyptian capital, etc., do precisely what I command you. The nature of the capital, with all its abominations prevented prayer offered in that city to G’d to become effective, as we know from 9,29 where Moses explains this to Pharaoh. G’d says: “I will command you from time to time to enter the capital in order to speak to Pharaoh.”

2ראה כל המופתים אשר שמתי בידך, pay careful attention each time you go to Pharaoh when you will be equipped with the power to perform all the miracles which I have placed at your disposal. You must perform these in exactly the manner I have instructed you.

3ועשיתם לפני פרעה, you will be successful in this only if you proceed exactly in the manner in which I instruct you, and in the presence of Pharaoh. When a creature sins against his Creator by doing either less or more than instructed to do, he will fail in what he set out to do. This rule did not only apply to Moses and Aaron when they set out to perform miracles, but it is a general rule applicable to all of G’d’s commandments, and this is why the Torah spelled this out in Deuteronomy 4,2 writing “do not add or subtract from all the commandments which I have commanded you.”

4ואני אחזק את לבו, for if he could not endure the plagues he would let the Israelites go, not because he was finally humbling himself before the Lord and do His will, but only in order to get relief from the plagues. This was not a good enough reason to grant him relief; therefore G’d reinforced his natural obstinacy.