We normally quote Rom. 15:4 in application of the whole Old Testament. This is good, but it is also good to take a look at what OT Psalm that Paul quoted in the previous verse that he was refering to (15:3 = Psa. 69:9b). How does Psa. 69:9b help us have hope and help the Roman church have a good attitude towards each other (Rom. 15:7).