Charles Ellicott Commentary Exodus 9:4

"And Jehovah shall make a distinction between the cattle of Israel and the cattle of Egypt; and there shall nothing die of all that belongeth to the children of Israel." — Exodus 9:4 (ASV)

The Lord shall sever. — Compare 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.