This shall be the service of the sons of Kohath in the tabernacle of the congregation, about the most holy things:

Jamieson-Fausset-Brown Bible Commentary

4-15. This 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.

Matthew Henry's Concise Commentary

4:4-20 The Kohathites were to carry the holy things of the tabernacle. All the holy things were to be covered; not only for security and respect, but to keep them from being seen. This not only marked the reverence due to holy things, but the mystery of the things signified by those types, and the darkness of the dispensation. But now, through Christ, the case is altered, and we are encouraged to come boldly to the throne of grace.