Or HaChaim on Exodus 6:4/אור החיים על שמות ו׳:ד׳

1 וגם הקימותי את בריתי, "And I also maintain My covenant, etc." G'd may have added here (according to our previous commentary) that even if He had already fulfilled His covenant with the patriarchs, they still would not have become privy to the degree of revelation that Moses had already now become privy to.

2Put simply, G'd explains to Moses in our verse that He owed the patriarchs something over and beyond His having revealed Himself to the patriarchs as the אל שדי. That something was the fulfilment of His promise/oath that the people of Israel would be redeemed from bondage. The time had now arrived for that promise to be kept; G'd also explained to Moses why he had to wait for the redemption somewhat longer, i.e. that G'd now invoked His מדת הרחמים. G'd was afraid that Moses would not have the emotional fortitude to pursue his mission with all the necessary vigour when he saw Pharaoh's obstinacy, or when he would face the pursuing Egyptian cavalry behind him and the sea in front; G'd therefore explained to Moses that the purpose of the redemption was not only the Exodus but also the conquest of the Holy Land.

3את ארץ מגוריהם, "the land in which they (the patriarchs) used to sojourn." Why did the Torah not content itself with describing the land the patriarchs had lived in as "the land of Canaan?" Why did the words את ארץ מגוריהם have to be added? Perhaps the Torah wanted to make us aware that the status of being aliens (which was part of the period of 400 years G'd had spoken to Abraham about) had already commenced during the lifetime of the patriarchs. This would have a great bearing on the timing of the Exodus. Still, this would leave open why the Torah had to use the word את twice when referring to that land. Use of that word creates the impression that the land of Canaan and the land in which the patriarchs sojourned were two different countries. Perhaps the Torah wanted to include the land of the Philistines [the coastal strip including Gaza, seeing that the Philistines were not of Canaanitic descent Ed.]. I have already explained this on Genesis 26,3.