John Calvin Commentary Numbers 4:34

"And Moses and Aaron and the princes of the congregation numbered the sons of the Kohathites by their families, and by their fathers` houses," — Numbers 4:34 (ASV)

And Moses and Aaron and the chief of the congregation. Another exception is added, namely, that no one should be received unless they were free from all defect and blemish. For we have seen elsewhere that those who were blind, lame, or defective in any part of their body were excluded from the tabernacle. This was to prevent their disfigurement from producing contempt and also so that they might be admonished by this external sign to preserve themselves more diligently from all spiritual defilement. Therefore, those are said to enter into the sanctuary who are fit to exercise the priesthood; and for this reason, the expression “for the service”⁴²⁶ is added.

Since the inequality (of their charges) might have been the source of envy, God’s authority is asserted at the end of the chapter. There, Moses records that he was only acting ministerially and that he distributed the offices among them according to God’s command.

⁴²⁶ “For the work.” — A. V. Ver. 35. Ver. 35.