Rev 9:13 Then the sixth angel sounded: And I heard a voice from the four horns of the golden altar which is before God,
"Horns" represent kingship (Dan 7-8), power to save (Ex 29:12) or destroy (Ex 21:14), "four" completeness (Rev 4:6-8), hence the voice sounding from the four horns of the altar before God symbolizes it has God's authority and everything in this vision is by God's decree, not the independent action of its actors.
14 saying to the sixth angel who had the trumpet, "Release the four angels who are bound at the great river Euphrates."
Compare:
6 And now you know what is restraining, that he may be revealed in his own time.
7 For the mystery of lawlessness is already at work; only He who now restrains will do so until He is taken out of the way.
8 And then the lawless one will be revealed, whom the Lord will consume with the breath of His mouth and destroy with the brightness of His coming.
(2Th 2:6-8 NKJ)
God unleashes the lying spirits that cause Babylon the Great to apostatize (fall) from all that is called God, to a new spiritistic religion of the antichrist. This kills them spiritually and physically.
NKJ Jeremiah 51:1 Thus says the LORD: "Behold, I will raise up against Babylon, Against those who dwell in Leb Kamai, A destroying wind. (Jer 51:1 NKJ)
The "Athbash" "Leb Kamai" is a cipher that reverses the order of spelling to indicate something more than Chaldea is meant, namely, "Babylon the Great."
The four angels are symbolic, not actual angels just as the four horses of the apocalypse are not actual horses, they symbolize what is being released will go out in every direction (Rev 6).
All not having God's name on their forehead (Rev 9:4; Mat 24:24; 2 Thess 2:13f) in Leb Kami Babylon the Great are the targets.
2 And I will send winnowers to Babylon, Who shall winnow her and empty her land. For in the day of doom They shall be against her all around.
3 Against her let the archer bend his bow, And lift himself up against her in his armor. Do not spare her young men; Utterly destroy all her army.
4 Thus the slain shall fall in the land of the Chaldeans, And those thrust through in her streets.
5 For Israel is not forsaken, nor Judah, By his God, the LORD of hosts, Though their land was filled with sin against the Holy One of Israel."
6 Flee from the midst of Babylon, And every one save his life! Do not be cut off in her iniquity, For this is the time of the LORD'S vengeance; He shall recompense her. (Jer 51:2-6 NKJ)
Verse 6 shows this occurs when God commands His people come out of Babylon:
4 And I heard another voice from heaven saying, "Come out of her, my people, lest you share in her sins, and lest you receive of her plagues.
5 "For her sins have reached to heaven, and God has remembered her iniquities.
6 "Render to her just as she rendered to you, and repay her double according to her works; in the cup which she has mixed, mix double for her.
7 "In the measure that she glorified herself and lived luxuriously, in the same measure give her torment and sorrow; for she says in her heart,`I sit as queen, and am no widow, and will not see sorrow.'
8 "Therefore her plagues will come in one day-- death and mourning and famine. And she will be utterly burned with fire, for strong is the Lord God who judges her. (Rev 18:4-8 NKJ)
15 So the four angels, who had been prepared for the hour and day and month and year, were released to kill a third of mankind.
God's decree is for this precise time, no more...no less, the scope of its destruction is limited "a third of mankind" is killed, not all.
16 Now the number of the army of the horsemen was two hundred million; I heard the number of them.
"Two hundred million" (δισμυριάδες μυριάδων) understates their number, μυριάς (“ten thousand”) designates an incalculable immensity, figurative of an innumerable, indefinite host (Gen. 24:60; Lev. 26:8; Num. 10:35[36]; Deut. 32:30; 33:2, 17; 1 Kgs. 18:7–8; 21:12[11]; Ps. 3:7[6]; Cant. 5:10; Sir. 47:6; Mic. 6:7; Dan. 7:10; in Ps. 90(91):7 LXX.
17 And thus I saw the horses in the vision: those who sat on them had breastplates of fiery red, hyacinth blue, and sulfur yellow; and the heads of the horses were like the heads of lions; and out of their mouths came fire, smoke, and brimstone.
This is a vision (ὅρασις) like Daniel's (4, 5, 8 etc), therefore these are symbols, not descriptions of actual objects. The horses are not technology, human or angelic...they symbolize an army.
These demon spirits are God's army of unholy myriads He sends to consume children of the Devil.
τοὺς καθημένους ἐπ᾽ αὐτῶν "those sitting (middle voice) on them" indicates the riders are affected by the action, not initiating it. God is in complete control, the demons are His helpless victims, they are now being forced to destroy their own followers.
All three colors of the breastplates and their phenomena occur in a volcano; hence, this is symbolic of hell. While breastplates of righteousness (Is 59:17; Eph 6:14) faith and love (1 Th 5:8) exist, these are breastplates of destruction...all killed by this demonic army are cast into the lowest hell to await judgment day.
Just as the heads of the tails injure (vs. 19), these "heads of lions" are ruthlessly efficient killing, "eager to tear...ambush" (Ps 17:12) to devour (1 Pet 5:8),
The "fire, smoke and brimstone" out of their mouths is the judgment of God against the slain (Rev 14:10)
18 By these three plagues a third of mankind was killed-- by the fire and the smoke and the brimstone which came out of their mouths.
Given the symbolic nature of this vision, "a third" symbolizes God's restraint, it could have been much worse.
19 For their power is in their mouth and in their tails; for their tails are like serpents, having heads; and with them they do harm.
This symbolizes all not consumed by the fire from their mouths, are killed by venom. This demonic killing horde is terrifyingly efficient, it kills those in front, and any it missed, as it passes through the ranks.
This isn't merely physical death, its spiritual also. These rise up in the Day of Christ only to be cast head long into the lake which burns with fire and sulphur.
20 But the rest of mankind, who were not killed by these plagues, did not repent of the works of their hands, that they should not worship demons, and idols of gold, silver, brass, stone, and wood, which can neither see nor hear nor walk.
21 And they did not repent of their murders or their sorceries or their sexual immorality or their thefts. (Rev 9:13-21 NKJ)
This confirms the connection is to Babylon the Great and its great spiritistic falling away from all previous beliefs in God, to follow the "god of forces" (Dan 11:38f) the demon spirits preached:
And he cried mightily with a loud voice, saying, "Babylon the great is fallen, is fallen, and has become a dwelling place of demons, a prison for every foul spirit, and a cage for every unclean and hated bird! (Rev 18:2 NKJ)
