What are "Arba'ah Malchin"?
Rashi: They are four kingdoms.
Why does it say "Yekumun Min Ar'a"?
Malbim: When Kimah is followed by [Min, or the prefix] Mem, he rose to leave, like "va'Yakumu mi'Sham ha'Anashim." The angel informed him that he did not see the four kingdoms as they stand. If so, he should not have seen Bavel, for its time already passed; it was time for it to depart! Rather, he saw them as they will depart and cease ? how they will depart from the land. Therefore, he saw also Bavel ? its wings shed their feathers, and it was lifted from the ground. The angel did not explain to him about the four kingdoms, only that the vision is about their departure, and the establishing of Malchus Shamayim at the end.
