4 Nadab and Abihu died before Yahweh, when they offered strange fire before Yahweh, in the wilderness of Sinai, and they had no children. Eleazar and Ithamar ministered in the priest’s office in the presence of Aaron their father.

Matthew Henry's Commentary on Numbers 3:4

Commentary on Numbers 3:1-13

(Read Numbers 3:1-13)

There was much work belonging to the priests' office, and there were now only Aaron and his two sons to do it; God appoints the Levites to attend them. Those whom God finds work for, he will find help for. The Levites were taken instead of the first-born. When He that made us, saves us, as the first-born of Israel were saved, we are laid under further obligations to serve him faithfully. God's right to us by redemption, confirms the right he has to us by creation.