John Wesley on Genesis 15:16

They shall come hither again - Hither to the land of Canaan, wherein thou now art. The reason why they must not have the land of promise in possession till the fourth generation, is because the iniquity of the Amorites was not yet full. The righteous God has determined, that they shall not be cut off till they are arrived to such a pitch of wickedness; and therefore till it come to that, the seed of Abram must be kept out of possession.