Or HaChaim on Deuteronomy 4:1/אור החיים על דברים ד׳:א׳

1 ועתה ישראל שמע אל החוקים, "And now, O Israel, hearken to the statutes, etc." Perhaps Moses referred to 2 separate incidents which had happened to him personally (when he had been remiss.) The first instance was the matter of speaking to the rock when Moses had struck the rock instead in his eagerness to fulfil G'd's instructions. The second instance was the matter of Zimri publicly cohabiting with the Midianite princess Cosbi when Moses had forgotten the ruling that anyone sincerely jealous of such a desecration of the name of the Lord is to execute the sinner forthwith without trial. On that occasion too, Moses had missed an opportunity to sanctify the name of the Lord as we have explained in that context. Even though Pinchas had articulated this halachah in front of Moses at the time (compare Sanhedrin 82) Moses had answered him: "he who reads the letter must execute what is written therein." As a result, Pinchas acquired the merit of the covenant of peace which in truth had been reserved for Moses himself.

2It is possible that Moses' reason for failing to act correctly had been that "his hands were weakened" when he had acted rashly in hitting the rock instead of speaking to it, all because he had not taken the time to reflect on what G'd's proper instructions had been. It is also possible that he felt that the rule קנאים פוגעים בו, that people jealous on behalf of G'd must execute such a sinner, applied only to someone who had seen the actual sin being performed not to someone who had heard about it second-hand. This is why he did not think he had the right to instruct Pinchas to carry out the deed. As a result "he sat on it," and did nothing contrary to his immediate response to hitting the rock when G'd had instructed him to take along his staff at the time. In view of these two failures on his part, Moses now said to the people: "hearken to the statutes, i.e. the laws which lack rationale such as the commandment to speak to the rock at the time. As far as Moses was concerned that instruction was like a חוקה for him, something he did not understand. Moses continued to speak of משפטים, social laws, i.e. the kind of laws which appear to contain an element of social justice such as the law to execute Jews who have sexual intercourse with Gentiles. Both Moses' failures related to one or the other of these types of commandments; this prompted him to warn the Israelites specifically not to become guilty of the mistakes he had made himself, though inadvertently, and which were the reason he suffered from the decree of not being allowed to enter the Holy Land.

3Moses concluded this exhortation with the plea "so that you will live and come and take possession of the land which the Lord your G'd is about to give to you." He implied that what happened to him should not happen to them. He added: לא תוסיפו על הדבר, "do not add to the word, etc." This too was a warning to the Israelites that something that happened to him should not happen to them. Torat Kohanim at the beginning of Parshat Acharey Mot (page 1138) describes Rabbi Eleazar ben Azaryah who illustrated the difference in impact between two different kinds of warnings. The first example had a doctor tell his patient not to eat cold foods nor to sleep on a cold floor. The second doctor gave the same instructions but added the warning that not heeding his advice would result in the patient's death just as another patient who had ignored this warning had died. The Rabbi pointed out that the warning of the second physician was taken much more seriously. Moses is compared to the second physician as he could tell the people from his own experience that ignoring the warning had proved fatal for him. He had deviated, i.e. exceeded his instructions by not talking to the rock but hitting it.

4ולא תגרעו ממנו, "neither shall you diminish from it;" in this instance Moses referred to his failure to do what was required in the matter of Zimri. His failure to demand that someone execute Zimri had been equivalent to his "diminishing" from one of the social laws of the Torah. Moses said ולא תגרעו, "you must not diminish," to indicate that even if by diminishing a law of the Torah such as the בועל ארמי your intention is to save someone's life by your inaction.