Albert Barnes Commentary Psalms 118:4

"Let them now that fear Jehovah say, That his lovingkindness [endureth] for ever." — Psalms 118:4 (ASV)

Let them now that fear the Lord say ... - Compare (Psalms 115:11). All that worship God are interested in what God has done for me. It is a manifestation of the divine character which should cheer them. They are called, therefore, to unite with the author of the psalm in praise and thanksgiving, not merely from sympathy with him, but because great truths of religion had been illustrated, in his case, which were of as much importance to them as to him.