Here is, 1. The nomination of the cities of refuge on that side Jordan where Israel now lay encamped. Three cities were appointed for that purpose, one in the lot of Reuben, another in that of Gad, and another in that of the half tribe of Manasseh, Deu 4:41-43. What Moses could do for that people while he was yet with them he did, to give example to the rulers who were settled that they might observe them the better when he was gone. 2. The introduction to another sermon that Moses preached to Israel, which we have in the following chapters. Probably it was preached the next sabbath day after, when the congregation attended to receive instruction. He had in general exhorted them to obedience in the former chapter; here he comes to repeat the law which they were to observe, for he demands a universal but not an implicit obedience. How can we do our duty if we do not know it? Here therefore he sets the law before them as the rule they were to work by, the way they were to walk in, sets it before them as the glass in which they were to see their natural face, that, looking into this perfect law of liberty, they might continue therein. These are the testimonies, the statutes, and the judgments, the moral, ceremonial, and judicial laws, which had been enacted before, when Israel had newly come out of Egypt, and were now repeated, on this side Jordan, Deu 4:44-46. The place where Moses gave them these laws in charge is here particularly described. (1.) It was over-against Beth-peor, an idol-temple of the Moabites, which perhaps Moses sometimes looked towards, with a particular caution to them against the infection of that and other such like dangerous places. (2.) It was upon their new conquests, in the very land which they had got out of the hands of Sihon and Og, and were now actually in possession of, Deu 4:47. Their present triumphs herein were a powerful argument for obedience.

And this is the law which Moses set before the children of Israel. Not the law concerning the cities of refuge, but the law of the ten commands repeated in the following chapter; so Jarchi remarks,"this which he should set in order after this section;''as he does in the next chapter, where he repeats in order the ten precepts, and makes observations on the manner of the delivery of them, and urges obedience to them.
And this is the law which Moses set before the children of Israel. Not the law concerning the cities of refuge, but the law of the ten commands repeated in the following chapter; so Jarchi remarks,"this which he should set in order after this section;''as he does in the next chapter, where he repeats in order the ten precepts, and makes observations on the manner of the delivery of them, and urges obedience to them.
Deuteronomy 4:45

deu 4:45
deu 4:45
deu 4:45These are the testimonies, and the statutes, and the judgments,.... The laws, moral, ceremonial, and judicial, delivered in the following chapters; which are renewed, repeated, and explained: which Moses spake unto the children of Israel, after they came forth out of Egypt; in the third month after they came from thence these laws were delivered to him at Mount Sinai, and he declared them to them; and now afresh, near forty years after, repeated them to them in the plains of Moab.

this is the law which Moses set before the children of Israel--This is a preface to the rehearsal of the law, which, with the addition of various explanatory circumstances, the following chapters contain.