Charles Ellicott Commentary Numbers 4:2

"Take the sum of the sons of Kohath from among the sons of Levi, by their families, by their fathers` houses," — Numbers 4:2 (ASV)

Of the sons of Kohath ... — Kohath appears to have been the second son of Levi (Numbers 3:17), but the Kohathites here stand first because Moses and Aaron belonged to them, and it was their office to bear the Ark.