Psalms 138:4,5 - Ellicott's Commentary On The Whole Bible

The general sense of these verses is plain, though there are slightly different ways of understanding the expressions. The psalmist imagines that the word or promise, which has been so abundantly fulfilled, will, by its performance, convince all the kings of the earth, and bring them in confession and praise to Jehovah. For a Hebrew the expression “hear the words of Thy mouth,” referring in this instance immediately back to Psalms 138:2, was synonymous with “see Thy wonders,” since for them “God spoke and it was done.”