Albert Barnes Commentary Psalms 108:4

"For thy lovingkindness is great above the heavens; And thy truth [reacheth] unto the skies." — Psalms 108:4 (ASV)

For thy mercy ... - This is taken from Psalms 57:10. The only change is in the expression above the heavens, instead of unto the heavens. The sense is essentially the same. The particular idea here, if it differs at all from the expression in Psalms 57:1-11, is that the mercy of God seems to “descend” from heaven upon man, or “comes down” from on high.