God is within her; she will not be moved. God will help her when morning dawns.
This song was sung in Jerusalem, and "her" is the city of God, the holy place with the temple. The verses around it describe seas roaring, mountains shaking, and armies closing in. Jerusalem has no great river, so the quiet stream in verse 4 stands out. "When morning dawns" points to the early hour when help arrived.
Psalm 46. A song about God as a safe place when everything around is shaken.
Read all of Psalm 46