Revelation 9:13-15 

Revelation 9:13-15
And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God, Saying to the sixth angel which had the trumpet, Loose the four angels which are bound in the great river Euphrates.

In Revelation 9:14-15, we're told about four angels who are bound in the river Euphrates. These verses tell us a few things. There's a geographical location involved, and these four angels are evil, as they are bound, which means they're being held against their will.

We should not confuse these four angels with the four Angels of Revelation 7, as they're "standing on the four corners of the earth." Therefore, they're not bound, and they're linked to the righteous Sealing of God's Saints that transpires before the Tribulation begins.

So on one hand, you have the four angels in the Euphrates who are bound, and not released until the Sixth Trumpet sounds. On the other, you have the four Angels of Revelation 7 that unleash the Four Winds, which initiates the Seven Trumpets, and later, the Tribulation.