The four horns that replaced the broken one represent four kingdoms that will rise from that nation but will not have the same power.
The great horn, says he, which was between his eyes was the first king, and when it was broken, four others sprang up. Alexander, as we have mentioned, perished in the flower of his age, and was scarcely’ thirty years old when he died, through the influence of either poison or disease.
Here we have, I. Daniel's earnest desire to have this vision explained to him (Dan 8:15): I sought the meaning. Note, Those that rightly know the things of God cannot but desire to know more and more of them, and to be led further into the mystery of them; and those that would find the meaning of what they have seen or heard from God...
Commenting on Daniel 8:15-27
Now that being broken,.... That is, the great horn Alexander, the first king of the Grecian monarchy; whose death, either by drunkenness, or by poison, is here expressed by being "broken". The sense is, he being dead, or upon his death, whereas four stood up for it; four horns rose up in the room and stead of the great one broken; see Dan 8:8 these...