The Lord shall sever. — Comp. Exodus 8:22. Apparently Israel had been subjected to the first, second, and third plagues, which caused annoyance only, and not loss. Their exemption began with the fourth plague, and then probably continued without intermission, though it is not always mentioned.
Related Commentaries of Exodus 9:4
Exodus 9:4
4 And the LORD shall sever between the cattle of Israel and the cattle of Egypt: and there shall nothing die of all that is the children's of Israel.