Who are the three kings?
Rashi citing Seder Olam 28: Koresh, Achashverosh and Daryavesh [ha'Parsi], who built the Bayis.
Refer to 11:2:2:2-5 and the notes there.
Who is the fourth?
Rashi, Ibn Ezra citing Rav Sadya Gaon: He is fourth from Madai (Daryavesh ha'Midi preceded the three kings of Paras).
Rashi citing Yosef ben Guryon: Koresh had a son Mevishah who ruled before Achashverosh 1 .
Ibn Ezra citing R. Moshe: There were six kings – Daryavesh ha'Midi, Koresh, Achashverosh, Daryavesh ha'Parsi, Artachshashta, who commanded not to build Ir ha'Kodesh, Daryavesh ha'Parsi, who commanded to build, and the sixth was Artachshashta ha'Ashir, whom Alexander killed. Ezra and Nechemyah ascended in his days. The verse does not count [Daryavesh ha'Midi and] Koresh, only future kings. 2
Ibn Ezra: There were five kings – Achashverosh is the Persian name for Artachshashta (the name in Leshon Kasdim). The Sitna (prosecution) at the beginning of Artachshashta's reign was not to build the Beis ha'Mikdash, after he initially authorized building. (Haman's Sitna was later.) Due to it, he retracted, like Koresh retracted. If Artachshashta was Koresh [who is Daryavesh], it would have said 'Hu Koresh Hu Daryavesh Hu Artachshashta 3 . Only three were after Koresh - Achashverosh, who is the first Artachshashta, Daryavesh ha'Parsi, and Artachshashta who reigned many years – "Shmas Sheloshim u'Shetayim l'Artachshashta" (Nechemyah 5:14); he is fourth from Koresh, that this Nevu'ah was in his days.
Malbim: There are three after Koresh/ Including Koresh, there are four.
I.e. Achashverosh was the third, and Daryavesh ha'Parsi was the fourth king of Paras. (PF)
Ibn Ezra: If the verse says that there will be [only] three more kings, how is there a fourth? Also, Koresh ruled three years, and Achashverosh 14. If so, when did the first Artachshashta rule, who commanded not to build the Bayis? (There were only 18 years between Belshatzar and when they began building! – PF)
Rosh Hashanah 3b equates them; Ibn Ezra rejects this, for no verse says so. However, he equates Koresh and Daryavesh, even though no verse equates them! He says that this Nevu'ah was in Koresh's days – it was in Daryavesh's first year (verse 1)! We fixed what seems to be a printing mistake in Ibn Ezra. (PF)
What is the meaning of "uch'Chezkaso v'Ashro Ya'ir ha'Kol Es Malchus Yavan"?
Rashi: Daryavesh [ha'Parsi], amidst his strength and wealth, will arouse his entire kingdom to fight Malchus Yavan.
Malbim: He will arouse all his soldiers to fight Malchus Yavan. Yavan did not have one king, only judges over the land.
