ועבדום וענו אותם. כנגד במה אדע וגו': ועבדום, וענו אותם, “they will serve them; and they (the masters) will oppress them.” This was the answer to Avraham’s question: במה אדע כי אירשנה, “how will I know that I will indeed inherit it (the land) even vicariously,” in verse 8, where Avraham asked for some kind of guarantee that G–d’s promise to him would not be negated by his descendants forfeiting their claim to its fulfillment.
ארבע מאות שנה. כנגד ד' תיבות. ד"א במ"ה עולה ארבע מאות בא"ת ב"ש. ועוד מסיק בפרק ראשון מנדרים מפני מה נענש אברהם שנשתעבדו בניו מפני שהפריד בני אדם מליכנס תחת כנפי השכינה שנא' תן לי הנפש: ארבע מאות שנה, “for four hundred years.” This is a separate revelation from G–d to Avraham. Another interesting explanation is that the numerical value of the letters in Avraham’s question במה adds up to 400 when you use the system known as א-ת, ב-ש, ג-ר, i.e. substituting the letters commencing by reading the alphah-bet backwards. Furthermore, the Talmud in tractate Nedarim folio 32, comes to the conclusion that the Jewish people had to suffer the penalty for their founding father having asked G–d for proof that His promise would come true, [although seeing that he had been unable to have children until that time would seem to us as a reasonable question. Ed.] Another reason given is that when Avraham agreed to the King of Sodom to have his citizens whom he had taken prisoner restored to him, he is punished for missing out on a chance to convert these people to monotheism. (Compare Genesis 14,21)