On this wise (houtōs). A free quotation from Gen 15:13.
Should sojourn (estai paroikon). Shall be a sojourner, Paroikos (para, beside, oikos, home), one dwelling near one’s home, but not of it, so a stranger, foreigner, old word, often in lxx, temporary residence without full rights of citizenship (Act 7:29; Act 13:17), and descriptive of Christians (Eph 2:19; 1Pe 1:17; 1Pe 2:11).
In a strange land (en gēi allotriāi). In a land not one’s own, that belongs to another, alien as in Mat 17:25., which see.
Four hundred years (etē tetrakosia). Accusative of duration of time. As in Gen 15:13, but a round number as in Exo 12:40 the time is 430 years. But in Gal 3:17 Paul, following the lxx in Exo 12:40, takes the 430 years to cover the period in Canaan and the stay in Egypt, cutting the sojourn in Egypt to about half. Josephus gives it both ways. Hackett suggests two solutions, one that there were two ways of reckoning the period among the Jews with no way of settling it, the other that by the 430 years in Egypt the writers meant to include Canaan also as merely the preliminary to the period in Egypt.