The law comes four times in 28:4-9. Is it the law of Moses?

The chapter doesn't say whose law it means. Law in 28:4, 28:7 and 28:9 is torah (tap law), and the book has used the word for more than one teaching: a mother's, "don’t forsake your mother’s teaching" (Proverbs 1:8); the teaching of the voice that says "My son" (Proverbs 3:1); a father's, "Don’t forsake my law" (Proverbs 4:2), with the verb and noun of 28:4's forsake the law; and the teaching of the wise: "The teaching of the wise is a spring of life" (Proverbs 13:14). In chapter 28 it has no owner: "Those who forsake the law praise the wicked; but those who keep the law contend with them" (28:4), "Whoever keeps the law is a wise son" (28:7), and "He who turns away his ear from hearing the law, even his prayer is an abomination" (28:9). The next chapter has it once more: "one who keeps the law is blessed" (Proverbs 29:18). Among these sayings, 28:5 speaks of the LORD: "those who seek the LORD understand it fully" (28:5). The chapter says no more about whose law it is, and these notes don't decide. On 28:9, the notes on Proverbs 15 set it beside 15:8 and 15:29. These notes add only that the verse speaks of one who turns his ear away from hearing, and they apply it to no one's prayers today.