Genesis 15:15

And thou shalt be gathered to thy fathers, thy soul shall rest in peace, and thou shalt be buried in a good old age.

AND THOU SHALT COME TO THY FATHERS IN PEACE. And thou shalt not behold at this. Thus the language of Rashi.
This is not correct according to Rashi’s own interpretation, namely, that the decree, thy seed shall be a stranger in a land not their own, took effect as soon as Abraham had a child. Thus, soon after Isaac was born it states, And Abraham sojourned (‘vayagar’) in the land of the Philistines; has the same root letters as geir (stranger), thus suggesting that Abraham lived as a “stranger” in a land which was not his own. And Isaac sojourned in Gerar., 26:6. The verse however states, vayeshev (and he “dwelled”), and not vayagar. In Rashi a different verse is quoted: Sojourn (‘gur’) in this land. (Ibid., Verse 3.) Now gur has the same root as vayagar. (See preceding note.) Now if so, Abraham was also included in the decree! But the meaning of the verse, And thou shalt come to thy fathers in peace, is that “no punishment will come to you from Me even though I decree on your children punishments of servitude and affliction.”

ואתה תבא אל אבותיך בשלום, you will die in peace, i.e. you will not witness in your lifetime any of the upsetting developments of which I have spoken to you. As far as the word אבותיך is concerned, this is a common expression used by most people as a metaphor making death sound less negative. Expressions such as ויאסף אל עמיו, "he was gathered in to his people" in Genesis 49,33, or והאסף אל עמיך, "be gathered in to your people" in Deuteronomy 32,50 all express similar sentiments. Some commentators (quoted by Rashi) claim that the wording proves that Terach, Avraham's father had discarded idolatry and accepted monotheism prior to his own death. This is problematic, seeing that we find in Chronicles II 34,28 that kind Yoshiyahu is told by G'd הנני אוסיפך אל אבותיך, and we know that his father Amon died as an unrepentant sinner. תקבר בשיבה טובה, you will see sons and grandsons before you die.

ודור רביעי ישובו הנה, the fourth generation, starting with those who descended to Egypt, which was the place of their exile, will return to this their land that they migrated from. This prediction was fulfilled, seeing that Kehat who was one of Yaakov's grandsons went down to Egypt, and the children of Moses and Aaron who were the fourth generation were among those who entered the land of Canaan under the leadership of Joshua.