Albert Barnes Commentary Psalms 56:4

"In God (I will praise his word), In God have I put my trust, I will not be afraid; What can flesh do unto me?" — Psalms 56:4 (ASV)

In God I will praise his word - The meaning of this seems to be, "In reference to God—or, in my trust in God—I will especially give regard to his “word”—his gracious promise; I will make that the special object of my praise. In reflecting on the divine perfections, and in finding in them materials for praise, I will give special regard to his revealed truth—to what he has “spoken” as an encouragement to me. I will be thankful that he “has” spoken, and that he has given me assurances on which I may rely in times of danger."

The idea is that he would “always” find in God that which was the ground or foundation for praise, and that what called for special praise when meditating on the divine character was the word or promise which God had made to his people.

I will not fear what flesh can do unto me - This means what man can do to me. Compare the notes at Matthew 10:28; Romans 8:31–34; Hebrews 13:6.