"For whatever was written in former days was written for our instruction, that through endurance and through the encouragement of the Scriptures we might have hope." (Romans 15:4)

This has nothing to do with following OT law. The point Paul is is trying to make is that the OT is still full of wisdom and truth and can provide still provide encouragement and comfort when in need.

"All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work." (2 Timothy 3:16-17)

Same deal as before.

 Don't misunderstand why I have come. I did not come to abolish the law of Moses or the writings of the prophets. No, I came to accomplish their purpose." -Matthew 5:17

"So, my brothers and sisters, you also died to the law through the body of Christ, that you might belong to another, to him who was raised from the dead, in order that we might bear fruit for God. For when we were in the realm of the flesh, the sinful passions aroused by the law were at work in us, so that we bore fruit for death. But now, by dying to what once bound us, we have been released from the law so that we serve in the new way of the Spirit, and not in the old way of the written code." -Romans 7:4-6