A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
jos 4:1TWELVE STONES TAKEN FOR A MEMORIAL OUT OF JORDAN. (Jos 4:1-8)
the Lord spake unto Joshua, Take you twelve men--each representing a tribe. They had been previously chosen for this service (Jos 3:12), and the repetition of the command is made here solely to introduce the account of its execution. Though Joshua had been divinely instructed to erect a commemorative pile, the representatives were not apprised of the work they were to do till the time of the passage.
jos 4:4Joshua called the twelve men--They had probably, from a feeling of reverence, kept back, and were standing on the eastern bank. They were now ordered to advance. Picking up each a stone, probably as large as he could carry, from around the spot "where the priests stood," they pass over before the ark and deposit the stones in the place of next encampment (Jos 4:19-20), namely, Gilgal.
jos 4:6That this may be a sign among you--The erection of cairns, or huge piles of stones, as monuments of remarkable incidents has been common among all people, especially in the early and rude periods of their history. They are the established means of perpetuating the memory of important transactions, especially among the nomadic people of the East. Although there be no inscription engraved on them, the history and object of such simple monuments are traditionally preserved from age to age. Similar was the purpose contemplated by the conveyance of the twelve stones to Gilgal: it was that they might be a standing record to posterity of the miraculous passage of the Jordan.
jos 4:8the children of Israel did so as Joshua commanded--that is, it was done by their twelve representatives.
jos 4:9TWELVE STONES SET UP IN THE MIDST OF JORDAN. (Jos 4:9)
Joshua set up twelve stones . . . in the place where the feet of the priests . . . stood--In addition to the memorial just described, there was another memento of the miraculous event, a duplicate of the former, set up in the river itself, on the very spot where the ark had rested. This heap of stones might have been a large and compactly built one and visible in the ordinary state of the river. As nothing is said where these stones were obtained, some have imagined that they might have been gathered in the adjoining fields and deposited by the people as they passed the appointed spot.
they are there unto this day--at least twenty years after the event, if we reckon by the date of this history (Jos 24:26), and much later, if the words in the latter clause were inserted by Samuel or Ezra.
jos 4:10THE PEOPLE PASS OVER. (Jos 4:10-13)
the priests which bare the ark stood in the midst of Jordan--This position was well calculated to animate the people, who probably crossed below the ark, as well as to facilitate Joshua's execution of the minutest instructions respecting the passage (Num 27:21-23). The unfaltering confidence of the priests contrasts strikingly with the conduct of the people, who "hasted and passed over." Their faith, like that of many of God's people, was, through the weakness of nature, blended with fears. But perhaps their "haste" may be viewed in a more favorable light, as indicating the alacrity of their obedience, or it might have been enjoined in order that the the whole multitude might pass in one day.
jos 4:11the ark of the Lord passed over, and the priests, in the presence of the people--The ark is mentioned as the efficient cause; it had been the first to move--it was the last to leave--and its movements arrested the deep attention of the people, who probably stood on the opposite bank, wrapt in admiration and awe of this closing scene. It was a great miracle, greater even than the passage of the Red Sea in this respect: that, admitting the fact, there is no possibility of rationalistic insinuations as to the influence of natural causes in producing it, as have been made in the former case.
jos 4:12the children of Reuben . . . passed over armed before the children of Israel--There is no precedency to the other tribes indicated here; for there is no reason to suppose that the usual order of march was departed from; but these are honorably mentioned to show that, in pursuance of their promise (Jos 1:16-18), they had sent a complement of fighting men to accompany their brethren in the war of invasion.
jos 4:13to the plains of Jericho--That part of the Arabah or Ghor, on the west, is about seven miles broad from the Jordan to the mountain entrance at Wady-Kelt. Though now desert, this valley was in ancient times richly covered with wood. An immense palm forest, seven miles long, surrounded Jericho.
jos 4:14GOD MAGNIFIES JOSHUA. (Jos 4:14-24)
On that day the Lord magnified Joshua in the sight of all Israel--It appeared clear from the chief part he acted, that he was the divinely appointed leader; for even the priests did not enter the river or quit their position, except at his command; and thenceforward his authority was as firmly established as that of his predecessor.
jos 4:18it came to pass, when the priests that bare the ark . . . were come out of the midst of Jordan . . . that the waters of Jordan returned unto their place--Their crossing, which was the final act, completed the evidence of the miracle; for then, and not till then, the suspended laws of nature were restored, the waters returned to their place, and the river flowed with as full a current as before.
jos 4:19the people came up out of Jordan on the tenth day of the first month--that is, the month Nisan, four days before the passover, and the very day when the paschal lamb required to be set apart, the providence of God having arranged that the entrance into the promised land should be at the feast.
and encamped in Gilgal--The name is here given by anticipation (see on Jos 5:9). It was a tract of land, according to JOSEPHUS, fifty stadia (six and one-half miles) from Jordan, and ten stadia (one and one-fourth miles) from Jericho, at the eastern outskirts of the palm forest, now supposed to be the spot occupied by the village Riha.
jos 4:20those twelve stones, which they took out of Jordan, did Joshua pitch in Gilgal--Probably to render them more conspicuous, they might be raised on a foundation of earth or turf. The pile was designed to serve a double purpose--that of impressing the heathen with a sense of the omnipotence of God, while at the same time it would teach an important lesson in religion to the young and rising Israelites in after ages.