The four horns that replaced the broken one represent four kingdoms that will rise from that nation, but will not have the same power.

Daniel 7:6
Next, as I watched, another beast appeared. It was like a leopard, and on its back it had four wings like those of a bird. The beast also had four heads, and it was given authority to rule.

Daniel 8:8
Thus the goat became very great, but at the height of his power, his large horn was broken off, and four prominent horns came up in its place, pointing toward the four winds of heaven.

Daniel 8:21
The shaggy goat represents the king of Greece, and the large horn between his eyes is the first king.

Daniel 8:23
In the latter part of their reign, when the rebellion has reached its full measure, an insolent king, skilled in intrigue, will come to the throne.

Daniel 11:4
But as soon as he is established, his kingdom will be broken up and parceled out toward the four winds of heaven. It will not go to his descendants, nor will it have the authority with which he ruled, because his kingdom will be uprooted and given to others.

Zechariah 6:1
And again I lifted up my eyes and saw four chariots coming out from between two mountains--mountains of bronze.

Now that being broken, whereas four stood up for it, four kingdoms shall stand up out of the nation, but not in his power.