What does Isaiah 21:4 mean?

The prophet of the Lord paid a price for witnessing visions of future judgment. Isaiah has seen the horrendous things coming for Babylon. This judgement is likely in the form of the Assyrian army led by Sennacherib (Isaiah 21:1–2). The prophet has reported that his body is wracked with pain, comparing the torment to a woman in labor. The experience has left him staggered and bewildered, not wanting to see any more (Isaiah 21:3).

Now he adds that his heart falters from the terror. While the imagery of a "failing heart" often means spiritual weakness, this reference is physical. This vision of horror is causing Isaiahs' body to tremble and his heart to race. He is describing the abject terror of those who will experience what he has only witnessed in his visions. His empathy has brought him to his knees with the same suffering they will experience ahead of their actual destruction.

Isaiah says that he has longed for the twilight, but now that has changed. That might mean that he longed for relief from his vision but found none. It may mean that his visions typically came at night—and what he used to desire, he now dreads. It's also possible Isaiah wished for a "twilight" to fall on the enemy nations, but seeing the actual carnage they will endure has changed his perspective. Whatever Isaiah expected experience doesn't occur.