I think, if I remember correctly that Abraham was told that his progeny would be slaves in Egypt for four hundred years. As when using the phrase "40 days" in the story of Noah's Flood means, figuratively, "a long time," I'm pretty sure 400 years means "a really long time." In the Bible, I've learned from some resources of Jewish scholars that numbers used in the Bible mean something more than precise dates. The waters of the Bible are deep and wide with manifold dynamics Note that by dynamics I am using the MerriamWebster.com definition : "variation and contrast in force or intensity (as in music)." We tend to try to simplify the Bible so it is easily digested by, for example, little children.