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
exo 4:1MIRACULOUS CHANGE OF THE ROD, &c. (Exo. 4:1-31)
But, behold--Hebrew, "If," "perhaps," "they will not believe me."--What evidence can I produce of my divine mission? There was still a want of full confidence, not in the character and divine power of his employer, but in His presence and power always accompanying him. He insinuated that his communication might be rejected and he himself treated as an impostor.
exo 4:2the Lord said, . . . What is that in thine hand?--The question was put not to elicit information which God required, but to draw the particular attention of Moses.
A rod--probably the shepherd's crook--among the Arabs, a long staff, with a curved head, varying from three to six feet in length.
exo 4:6Put now thine hand into thy bosom--the open part of his outer robe, worn about the girdle.
exo 4:9take of the water of the river--Nile. Those miracles, two of which were wrought then, and the third to be performed on his arrival in Goshen, were at first designed to encourage him as satisfactory proofs of his divine mission, and to be repeated for the special confirmation of his embassy before the Israelites.
exo 4:10I am not eloquent--It is supposed that Moses labored under a natural defect of utterance or had a difficulty in the free and fluent expression of his ideas in the Egyptian language, which he had long disused. This new objection was also overruled, but still Moses, who foresaw the manifold difficulties of the undertaking, was anxious to be freed from the responsibility.
exo 4:14the anger of the Lord was kindled against Moses--The Divine Being is not subject to ebullitions of passion; but His displeasure was manifested by transferring the honor of the priesthood, which would otherwise have been bestowed on Moses, to Aaron, who was from this time destined to be the head of the house of Levi (Ch1 23:13). Marvellous had been His condescension and patience in dealing with Moses; and now every remaining scruple was removed by the unexpected and welcome intelligence that his brother Aaron was to be his colleague. God knew from the beginning what Moses would do, but He reserves this motive to the last as the strongest to rouse his languid heart, and Moses now fully and cordially complied with the call. If we are surprised at his backwardness amidst all the signs and promises that were given him, we must admire his candor and honesty in recording it.
exo 4:18Moses . . . returned to Jethro--Being in his service, it was right to obtain his consent, but Moses evinced piety, humility, and prudence, in not divulging the special object of his journey.
exo 4:19all the men are dead which sought thy life--The death of the Egyptian monarch took place in the four hundred and twenty-ninth year of the Hebrew sojourn in that land, and that event, according to the law of Egypt, took off his proscription of Moses, if it had been publicly issued.
exo 4:20Moses took his wife and his sons, and set them upon an ass--Septuagint, "asses." Those animals are not now used in the desert of Sinai except by the Arabs for short distances.
returned--entered on his journey towards Egypt.
he took the rod of God--so called from its being appropriated to His service, and because whatever miracles it might be employed in performing would be wrought not by its inherent properties, but by a divine power following on its use. (Compare Act 3:12).
exo 4:24inn--Hebrew, "a halting place for the night."
the Lord met him, and sought to kill him--that is, he was either overwhelmed with mental distress or overtaken by a sudden and dangerous malady. The narrative is obscure, but the meaning seems to be, that, led during his illness to a strict self-examination, he was deeply pained and grieved at the thought of having, to please his wife, postponed or neglected the circumcision of one of his sons, probably the younger. To dishonor that sign and seal of the covenant was criminal in any Hebrew, peculiarly so in one destined to be the leader and deliverer of the Hebrews; and he seems to have felt his sickness as a merited chastisement for his sinful omission. Concerned for her husband's safety, Zipporah overcomes her maternal feelings of aversion to the painful rite, performs herself, by means of one of the sharp flints with which that part of the desert abounds, an operation which her husband, on whom the duty devolved, was unable to do, and having brought the bloody evidence, exclaimed in the painful excitement of her feelings that from love to him she had risked the life of her child [CALVIN, BULLINGER, ROSENMULLER].
exo 4:26So he let him go--Moses recovered; but the remembrance of this critical period in his life would stimulate the Hebrew legislator to enforce a faithful attention to the rite of circumcision when it was established as a divine ordinance in Israel, and made their peculiar distinction as a people.
exo 4:27Aaron met him in the mount of God, and kissed him--After a separation of forty years, their meeting would be mutually happy. Similar are the salutations of Arab friends when they meet in the desert still; conspicuous is the kiss on each side of the head.
exo 4:29Moses and Aaron went--towards Egypt, Zipporah and her sons having been sent back. (Compare Exo 18:2).
gathered . . . all the elders--Aaron was spokesman, and Moses performed the appointed miracles--through which "the people" (that is, the elders) believed (Kg1 17:24; Jos 3:2) and received the joyful tidings of the errand on which Moses had come with devout thanksgiving. Formerly they had slighted the message and rejected the messenger. Formerly Moses had gone in his own strength; now he goes leaning on God, and strong only through faith in Him who had sent him. Israel also had been taught a useful lesson, and it was good for both that they had been afflicted.