Charles Ellicott Commentary Numbers 4:28

"This is the service of the families of the sons of the Gershonites in the tent of meeting: and their charge shall be under the hand of Ithamar the son of Aaron the priest." — Numbers 4:28 (ASV)

Under the hand of Ithamar. It appears from Exodus 38:21 that Ithamar had been actively involved in the construction of the Tabernacle. As the special duties of Eleazar, the eldest son of Aaron, had been already assigned (Numbers 4:16), so also are those of Ithamar, the younger son. These duties consisted in the superintendence of the Gershonites, as stated in this verse, and in the superintendence of the Merarites, as stated in Numbers 4:33.