What is the significance of this vision?
Radak: It is more explained than the first. He understood that the horns are to gore, like "v'Karnei Re'em Karnav Bahem Amim Yenagach Yachdav" (Devarim 33:17). However, he did not understand who are these horns.
Malbim: This explains the previous Nevu'ah "Od Tefutzenah Arai" (1:17).
What is the significance of four Keranos?
Targum Yonasan: They are four kingdoms. Radak ? i.e. Bavel, Paras and Yavan 1 . All of them harmed Yisrael, like the angel explained "Asher Zeru Es Yehudah Es Yisrael vi'Yrushalayim" (2).
Malbim: There are two meanings of Keren. (a) They are horns on animals that gore. (b) They are corners in the four directions of symmetrical images.
Some editions add in brackets 'u'Madai.' Seemingly, they should add 'Edom', which is one of the four. We can say that Madai is included in Paras! (PF)