Daniel 7:17 "These great beasts, which are four, are four kings, which shall arise out of the earth."
Daniel 7:6 "After this I beheld, and lo another, like a leopard"
After Medo-Persia comes Greece.
After Alexander the Macedonian, Philip's son, who came from the land of Kittim, had defeated Darius, king of the Persians and Medes, he became king in his place, having first ruled in Greece. (1 Maccabees 1:1)
Daniel 7:6 "After this I beheld, and lo another, like a leopard, which had upon the back of it four wings of a fowl; the beast had also four heads; and dominion was given to it."
Greece is conquering even faster with the four wings compared to Babylon with two.
Four heads because after Alexander died and his kingdom was divided among his four leading generals "Cassander, Lysimachus, Seleucus, and Ptolemy".
1843 Chart (By Charles Fitch and Apollos Hale)
1850 Chart (By Otis Nichols)