Why did John hear the voice coming from these four specific locations of the altar?

The golden altar before God, used for incense, presents the prayers of the saints to the Almighty. The blood on the four horns proclaims the blessed consequence of the Saviour’s atoning work. Saints on earth continue to cry out to God (see Revelation 6:10; Revelation 8:3–4). The Lord, in faithfulness to his promise, hears the petitions of his people. The command of Revelation 9:14, coming as it does from the four corners of the altar, reassures John (and his readers) that the Lord hears their cries for Jesus’ sake.