C. F. Keil and Franz Delitzsch on Leviticus 4:1

Lev 4:1

The Expiatory Sacrifices. - The sacrifices treated of in ch. 1-3 are introduced by their names, as though already known, for the purpose of giving them a legal sanction. But in ch. 4 and 5 sacrifices are appointed for different offences, which receive their names for the first time from the objects to which they apply, i.e., from the sin, or the trespass, or debt to be expiated by them: viz., חטּאת sin, i.e., sin-offering (Lev 4:3, Lev 4:8, Lev 4:14, Lev 4:19, etc.), and אשׁם debt, i.e., debt-offering (Lev 5:15-16, Lev 5:19); - a clear proof that the sin and debt-offerings were introduced at the same time as the Mosaic law. The laws which follow are distinguished from the preceding ones by the new introductory formula in Lev 4:1-2, which is repeated in Lev 5:14. This repetition proves that Lev 4:2-5:13 treats of the sin-offerings, and Lev 5:14-19 of the trespass-offerings; and this is confirmed by the substance of the two series of laws.