Four Hundred is the product of 8 and 50, and is a divinely perfect period.
From the fulfillment of the promise to Abraham in the birth of Isaac to the Exodus was a period of 400 years. Stephen mentions it as dating from “his seed” (Acts 7:6), and God dates it from the same point when He says to Abraham, “thy seed” (Genesis 15:13).*
* The structure of this verse rescues it from the way in which it is sadly misunderstood:—
A. “Know of a surety that thy seed shall be a stranger in a strange land that is not theirs,
B. and shall serve them;
B. and they shall afflict them
A. four hundred years.”
The structure places B and B in a parenthesis. A and A cover the whole time of the sojourn, while B and B refer to the service and sojourn during a parenthesis of unnamed length within that time. The structure of Acts 7:6 is exactly the same.