The rest of Deuteronomy 4 was a supplement to Moses, probably written later by Ezra when he compiled the Old Testament canon into its present form. It is written about Moses, rather and what he did after his speeches. Ezra writes,
41 Then Moses set apart three cities across the Jordan to the east, 42 that a manslayer might flee there, who unintentionally slew his neighbor without having enmity toward him in time past; and by fleeing to one of these cities, he might live; 43 Bezer in the wilderness on the plateau for the Reubenites, and Ramoth in Gilead for the Gadites, and Golan in Bashan for the Manassites.
These were the cities of refuge established east of the Jordan River, among the tribes of Reuben, Gad, and the half tribe of Manasseh. These cities were places where those guilty of involuntary manslaughter could live under protection of law until their liability ended with the death of the high priest (Num. 35:25). A full account of this law is found in Numbers 35, where we learn that three cities of refuge were to be established on each side of the Jordan River (Num. 35:14).
Ezra concludes by telling us that this “Second Law” called Deuteronomy was given “across the Jordan,” while Moses was yet alive. Obviously, it was written from the perspective of one who lived west of the Jordan River in the land of Canaan.
44 Now this is the law which Moses set before the sons of Israel; 45 these are the testimonies and the statues and the ordinances which Moses spoke to the sons of Israel, when they came out of Egypt, 46 across the Jordan, in the valley opposite Beth-peor, in the land of Sihon king of the Amorites who lived at Heshbon, whom Moses and the sons of Israel defeated when they came out of Egypt. 47 And they took possession of his land and the land of Og king of Bashan, the two kings of the Amorites, who were across the Jordan to the east, 48 from Aroer, which is on the edge of the valley of Arnon, even as far as Mount Sion (that is, Hermon), 49 with all the Arabah across the Jordan to the east, even as far as the sea of the Arabah at the foot of the slopes of Pisgah.