Psalms 126:4

These verses look forward to the mercies that were yet wanted. Those that had come out of captivity were still in distress, even in their own land (Neh 1:3), and many yet remained in Babylon; and therefore they rejoiced with trembling, and bore upon their hearts the grievances that were yet to be redressed. We have here, 1. A prayer for the perfecting of their deliverance (Psa 126:4): "Turn again our captivity. Let those that have returned to their own land be eased of the burdens which they are yet groaning under. Let those that remain in Babylon have their hearts stirred up, as ours were, to take the benefit of the liberty granted." The beginnings of mercy are encouragements to us to pray for the completing of it. And while we are here in this world there will still be matter for prayer, even when we are most furnished with matter for praise. And, when we are free and in prosperity ourselves, we must not be unmindful of our brethren that are in trouble and under restraint. The bringing of those that were yet in captivity to join with their brethren that had returned would be as welcome to both sides as streams of water in those countries, which, lying far south, were parched and dry. As cold water to a thirsty soul, so would this good news be from that far country, Pro 25:25. 2.

— Matthew Henry's Concise Commentary (public domain)