Devarim 4:41-43 record that Moshe designated three cites of refuge for the accidental murderer on the eastern side of the Jordan River. This designation is puzzling since it seems to interrupt Moshe's address to the people in chapters four and five. While most commentators (see Rashi on 4:41) understand that Moshe just designated the cities, and hence the designation can also be considered as part of Moshe's speech, still the designation differs from the remainder of chapter four where Moshe exhorted the people to follow all the laws. Why does the Torah record Moshe's designation of the cities of refuge in the end of chapter four? Why did Moshe not do this in Bemidbar 35:9-34 which record the laws of the cities of refuge? Also, Moshe could have designated the cities of refuge in chapter three, 3:1-17, where Moshe discussed the conquest of the land on the eastern side of the Jordan River.
Rashi (on 4:41) quoting from the Talmud (Makkot 10a) and the Midrash (Deuteronomy Rabbah 2:26) writes that Moshe did not want to miss an opportunity to perform a commandment. Thus, even though the cities of refuge on the eastern side of the Jordan River would not function until after the cities of refuge on the western side of the Jordan River were established, still at least Moshe could designate the future cities. This idea that Moshe would designate the cities even prior to their use is logical, yet this does not explain why the designation is recorded in chapter four.
The Ramban and Seforno (on 4:41) suggest a different rationale. They note that in chapter four Moshe had been exhorting the people to follow the laws and Moshe was about to teach numerous laws to the people. Accordingly, Moshe designated the cities to show the people that he was fulfilling all the commandments that he possibly could. This approach also makes sense, but why did Moshe choose this particular commandment concerning the cities of refuge to demonstrate his ardor to fulfill the commandments?
The Rashbam (on 4:41) offers a compelling reason why Moshe designated the cities of refuge. Bemidbar 35:9-34 record the laws of the cities of refuge, and Bemidbar 35:14 records that there were to be three cities of refuge on the eastern side of the Jordan River, and three cities of refuge on the western side of the Jordan River. The laws of the city of refuge are recorded again in the book of Devarim, 19:1-11, but in that section there is no mention of the cities on the eastern side of the Jordan River. The Rashbam explains that since Moshe referred to these cities in 4:41-43 there was no need to refer to them again in chapter 19. This explanation also makes sense, but again it does not explain the placement of 4:41-43 since Moshe could still have referred to the cities of refuge on the eastern side of the Jordan River in chapter 19 or any point prior to chapter 19.
While each approach on its own has some difficulties, all of them could be combined as follows. Moshe wanted to show his fulfillment of the laws and then he had to perform one commandment, and then he designated the cities of refuge on the eastern side of the Jordan River as an example of his performance of the law and in order to save him from mentioning them in chapter 19.
While this combination approach is possible, my guess is that the designation of the cities of refuge relate to the establishment of the covenant in the book of Devarim, which is the theme of the book of Devarim. How do the cities of refuge relate to the covenantal theme?
4:5 records, "See, I am teaching you laws and regulations as G-d has commanded me to do amid the land that you are entering to possess” (Fox, 1995, p. 865, translation). We see that the main emphasis of the covenant is the observance of the laws in the land of Israel, but what about those tribes, Reuven, Gad, and Menashe, that were going to live outside the land of Israel proper, were they also part of the covenant? The covenant was binding on the Jewish people before they reached the land of Israel, but maybe some people thought that once the people reached the land of Israel, then only those people living in Israel proper would be part of the covenant. Moshe's declaration of the cities of refuge in the eastern side of the Jordan River, outside of the land of Israel proper, is a statement that the laws and hence the covenant applied outside of the land of Israel proper. Specifically because the cities of refuge on the eastern side of the Jordan River would only function after the land of Israel was settled, Moshe's declaration was informing the tribes of Gad, Reuven and Menashe that they were part of the covenant even after the majority of the population had settled in the land of Israel. Accordingly, the designation of the cities of refuge could be considered as part of the introduction to the establishment of the covenant since it taught that the inhabitants of the East Bank of the Jordan River were also part of the covenant, and then chapter four of the book of Devarim is an appropriate place to record this designation.