Or HaChaim on Deuteronomy 4:40/אור החיים על דברים ד׳:מ׳

1 נתן לך כל הימים, "which He gives to you for all time." Seeing that Moses told the people to observe the Torah in order to enjoy long life, he was afraid that the people would interpret that by observing the commandments of the Torah G'd would lengthen their lifespan on earth. As a result of such reasoning the people might also argue that this was proof that just as man's life on earth is limited in time (compare Job 14,5 "his days are determined, You know the number of his months"), so possession of the Holy Land would also be subject to limitations in time. Moses therefore corrects any such impression by going on record that a) G'd would add years to the people's lives if they observed the Torah, and b) assuring them that G'd's gift of the land to the Jewish people is a gift in perpetuity. It follows that when he mentioned "in order that you will have a long life on the earth, etc," Moses meant that they would not die prematurely, just as we have been told in Deut. 11,21. As to the land itself, it will remain Israel's forever.