The loud cry sounds as soon as the fourth angel has finished his work. This has now been accomplished. The Two Witnesses have finished their prophecy in sackcloth...

And after these things I saw another angel come down from heaven, having great power; and the earth was lightened with his glory. And he cried mightily with a strong voice, saying, Babylon the great is fallen, is fallen, and is become the habitation of devils, and the hold of every foul spirit, and a cage of every unclean and hateful bird. For all nations have drunk of the wine of the wrath of her fornication, and the kings of the earth have committed fornication with her, and the merchants of the earth are waxed rich through the abundance of her delicacies. And I heard another voice from heaven, saying, Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues. (Revelation 18:1-4)

But the light of the Fourth Angel will shine ever brighter until the end, as a testimony against the fallen world (see Matthew 24:14):

We need to study the pouring out of the seventh vial [Revelation 16:17-21]. The powers of evil will not yield up the conflict without a struggle. But Providence has a part to act in the battle of Armageddon. When the earth is lighted with the glory of the angel of Revelation eighteen, the religious elements, good and evil, will awake from slumber, and the armies of the living God will take the field.—The S.D.A. Bible Commentary 7:983 (1899) {LDE 251.3}