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
num 4:2OF THE LEVITES' SERVICE. (Num. 4:1-49)
sons of Kohath, from thirty years old and upward--This age was specifically fixed (see on Num 8:24) as the full maturity of bodily energy to perform the laborious duties assigned them in the wilderness, as well as of mental activity to assist in the management of the sacred services. And it was the period of life at which John the Baptist and Christ entered on their respective ministries.
even unto fifty--The term prescribed for active duty was a period of twenty years, at the end of which they were exempted from the physical labors of the office, though still expected to attend in the tabernacle (Num 8:26).
all that enter into the host--so called from their number, the order and discipline maintained through their ranks, and their special duty as guards of the tabernacle. The Hebrew word, however, signifies also a station or office; and hence the passage may be rendered, "All that enter into the sacerdotal office" (Num 4:23).
num 4:4This shall be the service of the sons of Kohath, &c.--They are mentioned first, from their close connection with Aaron; and the special department of duty assigned to them during the journeyings of Israel accorded with the charge they had received of the precious contents of the tabernacle. But these were to be previously covered by the common priests, who, as well as the high priest, were admitted on such necessary occasions into the holy place. This was an exception to the general rule, which prohibited the entrance of any but the high priest. But when the cloud removed from the tabernacle, the sanctuary might be entered by the common priests, as to them was reserved the exclusive privilege of packing the sacred utensils; and it was not till the holy things were thus ready for carriage, that the Kohathites were allowed to approach.
num 4:5covering veil--the inner veil, which separated the holy from the most holy place. (See on Exo 36:35).
num 4:6covering of badgers' skins--(See on Exo 25:5). The covering, however, referred to was not that of the tabernacle, but one made for the special purpose of protecting the ark.
put in the staves--These golden staves were now taken out. (See on Exo 25:15, compared with Kg1 8:8). The Hebrew word rendered "put in," signifies also "dispose," and probably refers here to their insertion through the openings in the coverings made for receiving them, to preserve them from the touch of the carriers as well as from the influence of the weather. It is worthy of notice that the coverings did not consist of canvas or coarse tarpaulin, but of a kind which united beauty with decency.
num 4:7continual showbread--Though the people were in the wilderness fed upon manna, the sacred loaves were constantly made of corn, which was probably raised in small quantities from the verdant patches of the desert.
num 4:10a bar--or bier, formed of two poles fastened by two cross pieces and borne by two men, after the fashion of a sedan chair.
num 4:12instruments of ministry--the official dress of the priests (Exo 31:10).
num 4:13shall take away the ashes from the altar, &c.--The necessity of removing ashes from the altar plainly implies that sacrifices were offered in the wilderness (compare Exo 18:12; Exo 24:4), though that rebellious race seems frequently to have neglected the duty (Amo 5:25). No mention is made of the sacred fire; but as, by divine command, it was to be kept constantly burning, it must have been transferred to some pan or brazier under the covering, and borne by the appointed carriers.
num 4:15the sons of Kohath shall come to bear it, but they shall not touch any holy thing, lest they die--The mode of transport was upon the shoulders of the Levites (see on Num 7:9), although afterwards wheeled vehicles were employed (Sa2 6:3; Ch1 15:12). And it was allowed to touch the covering, but not the things covered, on the penalty of death, which was inflicted more than once (Sa1 6:19; Sa2 6:6-7). This stern denunciation was designed to inspire a sentiment of deep and habitual reverence in the minds of those who were officially engaged about holy things.
num 4:16to the office of Eleazar . . . pertaineth the oil for the light, and the sweet incense, &c.--He was charged with the special duty of superintending the squadron who were employed in the carrying of the sacred furniture; besides, to his personal care were committed the materials requisite for the daily service, and which it was necessary he should have easily at his command (Exo 29:38).
num 4:17Cut ye not off the tribe of the families of the Kohathites from among the Levites, &c.--a solemn admonition to Moses and Aaron to beware, lest, by any negligence on their part, disorder and improprieties should creep in, and to take the greatest care that all the parts of this important service be apportioned to the proper parties, lest the Kohathites should be disqualified for their high and honorable duties. The guilt of their death would be incurred by the superintending priest, if he failed to give proper directions or allowed any irreverent familiarity with sacred things.
num 4:24This is the service of the families of the Gershonites, &c.--They were appointed to carry "the curtains of the tabernacle"--that is, the goats' hair covering of the tent--the ten curious curtains and embroidered hangings at the entrance, with their red morocco covering, &c.
num 4:28their charge shall be under the hand of Ithamar the son of Aaron, &c.--The Levites were generally subject to the official command of the priests in doing the ordinary work of the tabernacle. But during the journeyings Eleazar, who was next in succession to his father, took the special charge of the Kohathites [Num 4:16], while his brother Ithamar had the superintendence of the Gershonites and Merarites [Num 4:33].
num 4:29As for the sons of Merari--They carried the coarser and heavier appurtenances, which, however, were so important and necessary, that an inventory was kept of them--not only on account of their number and variety, but of their comparative commonness and smallness, which might have led to their being lost or missing through carelessness, inadvertency, or neglect. It was a useful lesson, showing that God disregards nothing pertaining to His service, and that even in the least and most trivial matters, He requires the duty of faithful obedience.
num 4:34Moses and Aaron and the chief of the congregation numbered the sons of the Kohathites, &c.--This enumeration was made on a different principle from that which is recorded in the preceding chapter [Num 3:15]. That was confined to the males from a month old and upward, while this was extended to all capable of service in the three classes of the Levitical tribe. In considering their relative numbers, the wisdom of Divine Providence appears in arranging that, whereas in the Kohathites and Gershonites, whose burdens were few and easier, there were but about a third part of them which were fit for service; the Merarites, whose burdens were more and heavier, had above one half of them fit for this work [POOLE]. The small population of this tribe, so inferior to that of the other tribes, is attempted to be explained (see on Num 3:39).