Joshua 4:16

Command the priests that bear the ark of the testimony
In all places before, in this history, the ark is called the ark of the covenant, here the ark of the testimony, which signifies the same thing, the law; which was both the covenant between God and the people, and a testimony of his will unto them; the Septuagint version has both words:

that they come up out of Jordan;
where they stayed until all the people passed over, for the encouragement of them, and until they received this order.