Sforno on Exodus 32:13/ספורנו על שמות ל״ב:י״ג

1וכל הארץ הזאת אשר אמרתי אתן לזרעכם ונחלו, a reference to the fourth generation of the Emorite mentioned by G’d at the covenant between the pieces. (Genesis 15,16) Moses realises that this promise will not be fulfilled to his children except now.