Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
Numbers Chapter 4
About the most holy things - Omit "about." The sense is, "this is the charge of the sons of Kohath, the most holy things:" i. e. the ark of the covenant, the table of showbread, the candlestick, and the golden altar, together with the furniture pertaining thereto. It appears, from a comparison of Num 4:16, Num 4:28, Num 4:33, that the ministry of the Kohathites was superintended by Eleazar, the older of the two surviving sons of Aaron; and that of the two other families by Ithamar.
wholly of blue - Compare Exo 25:4 note. The third and external covering of the ark only was to be of this color. The table of showbread had Num 4:8 an outer wrapping of scarlet; the altar Num 4:13 one of purple.
Put in the staves - Probably, "put the staves in order." These were never taken out of the golden rings by which the ark was to be borne (see Exo 25:14-15), but would need adjustment.
to see when the holy things are covered - Render: to see the holy things for an instant. The expression means Iiterally" as a gulp," i. e. for the instant it takes to swallow.
enter in to perform the service - literally, as in the margin, "to war the warfare," or, as the same phrase in part is rendered, Num 4:3, "enter into the host to do the work." The language is military. The service of God is a sacred warfare (Num 8:24-25 marginal reading).
The Gershonites and Merarites are superintended by Ithamar, Aaron's younger son, who had already had the oversight of the tabernacle in its construction Exo 38:21. Thus, readily do the permanent offices of the leaders of the Israelite community spring out of the duties which, under the emergencies of the first year of the Exodus, they had been led, from time to time, to undertake.
By name ye shall reckon the instruments - Or, assign them to their bearers singly, and "by name." These "instruments" comprised the heavier parts of the tabernacle; and the order seems intended to prevent individual Merarites choosing their own burden, and so throwing more than the proper share on others.