"There is a river whose streams make glad the city of God, the holy place where the Most High dwells. God is within her, she will not fall; God will help her at the break of day."
In verse 4 we see a happy river flowing through the city of God in contrast to the raging waters of verses 2 and 3. We see that despite what chaos may be going on outside, those in God's kingdom experience peace. David, who wrote the psalm, would have also understood rivers to be places of abundance and provision. Imagine the stark contrast between the desert he and his men were fleeing through with the lush green foliage around the riverbanks and the fresh water for his men to drink. Not only is God's kingdom peaceful, but it is also abundant and full of life. In short, it is a paradise, an oasis in the midst of chaos.
It is this way because the Most High lives there and because He lives there, it will not fall. The phrase "God will help her at the break of day" can also be translated "will help her see the dawn", that is He will see her through the night safely. Traditionally, the place where God dwells is the Temple.
But the best part is this: You are the temple or "Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God?" (1 Cor 6:19) With God in Christ through the Holy Spirit, we have inner strength and peace when the world around us crumbles.