Albert Barnes Commentary Ezekiel 4:5

"For I have appointed the years of their iniquity to be unto thee a number of days, even three hundred and ninety days: so shalt thou bear the iniquity of the house of Israel." — Ezekiel 4:5 (ASV)

According to the number of the days - Or, “to be to you as a number of days (even as)” and so on. Compare the margin reference.

Some understand that these “days” were the years during which Israel and Judah sinned. For Israel, these date from Jeroboam’s rebellion to the time at which Ezekiel wrote (circa 390 years); and for Judah, from Josiah’s reformation.

But it seems more in accordance with the other “signs” to suppose that they represent not what had been, but what shall be. The whole number of years is 430 (Ezekiel 4:5–6), the number assigned long ago for the affliction of the descendants of Abraham (Genesis 15:13; Exodus 12:40).

The “forty years” apportioned to Judah (Ezekiel 4:6) bring to mind the 40 years spent in the wilderness. These were years not only of punishment but also of discipline and preparation for restoration. So, Ezekiel would suggest that the difference between the punishments of Israel and Judah is this: Israel's punishment would be of much longer duration with no definite hope of recovery, while Judah's would be imposed with the express purpose of the renewal of mercy.