Genesis 15:16

In the fourth generation they shall come hither.

Some hold that this means the same as four hundred years, but it is more likely to mean the four generation after the children of Israel went down into Egypt. Caleb was the fourth from Judah; Moses the fourth from Levi.

For the iniquity of the Amorites is not yet full.

Though the Amorites, the chief nation of Palestine, were wicked, they had not yet reached that extreme which would call for their destruction as unfit to cumber the earth. God's long suffering toward the wicked causes him to postpone his judgments until they have become so depraved that all hope of reformation is gone. God would not, therefore, now, displace the Amorites to make room for Israel.