Genesis 15:16

But in the fourth generation they shall return here, for the iniquity of the Amorites is not yet complete.” (Genesis 15:16 NKJV)

The Amorites were a people who were not following The Lord. They were doing evil and The Lord had a judgement against them, but He was going to allow another 400 years of evil to happen before He did anything about it. When God is criticized, (the very idea makes me shutter inside) He is accused of not caring enough, and allowing evil in this world, or He is accused of killing "innocent" lives when He calls for the annihilation of a whole people group. In this passage, we see a plan in the mind of God. He knows what He's doing. After all, He's God, and we're not.