Chapter four continues the account from chapter three. God had told Joshua that there needed to be a visible reminder of this event for the succeeding generations to look at and remember what God had done for them that day (Joshua 4:1-7). To accomplish this, God had each of the twelve tribes select one of their own men to retrieve a stone from the middle of the river bed, where the priests were standing with the ark. These twelve stones would be placed together into a pile on the west side of the Jordan, where the people entered the land, as a memorial. Whenever their descendants asked about it, the story could be handed down from one generation to the next of “how the water of the Jordan stopped flowing before the ark of the covenant of the LORD.” Again, the people obeyed, and at the time of Joshua’s writing, probably near the end of his life, the memorial was still standing.
The rest of the chapter finishes a couple of details introduced in earlier chapters. First, the tribes of Reuben, Gad, and Manasseh, who were staying on the east side of the river, followed through with their promise to send troops into Canaan, adding about 40,000 to the rest of the army that would be staying in the land (Joshua 4:10-14). Second, as soon as the priests stepped up onto dry land the river started to flow again, signifying God’s control over creation (Joshua 4:15-18). Third, the memorial was built as God had instructed (Joshua 4:19-24). God orchestrated these things exactly “so all the nations of the earth might recognize the LORD’s power and so [Israel] might always obey the LORD your God.”
Chapter four actually best ends with Joshua 5:1, which is a note confirming that what God intended actually happened. When the kings in Canaan heard about the Jordan River crossing, it did remind them of the Red Sea crossing, and “they lost their courage and could not even breathe for fear of the Israelites.” As noted in chapter two, it makes one wonder why they did not flee and evacuate rather than stand and be destroyed by Jehovah.