What does Revelation 16:4 mean?

The contents of the third bowl empty into the water supply sources. As a result, the water supply sources turn to blood—this is most likely a symbolic description, just as it was in Revelation 8:8–9. This is somewhat different than the second bowl judgment (Revelation 16:3), which more specifically described the putrid condition of ocean waters and how everything living in them died. The fact that this affects all of "the rivers and the springs of water" means that now all of the world's water—fresh or salt—has been corrupted. Earth, at this point, is in the midst of planetary death throes.

This bowl judgment is an intensification of the third trumpet judgment (Revelation 8:10–11). This third bowl judgment affects the water people use for drinking, bathing, washing clothes, and keeping plants, lawns, shrubs, and trees alive. Just as the second bowl judgment seems to have affected all ocean waters, so too does the third bowl judgment seem to pollute all of the world's fresh water.

When the nation of Israel rejected God and turned to idolatry by building and worshiping a golden calf, Moses ground the idol into powder and sprinkled it into the people's water source (Exodus 32:19–20). Perhaps the gold powder temporarily changed the color of the water. This was God's way of making the people see and taste the putrefying effects of their sin.

Similarly, the beast and his followers had shed the blood of the saints and now would taste blood. Their sin would return to haunt them. This is precisely what an angel will declare in the next few verses.