Part of the reason why God's righteous plan is via faith is so that "it might be according to grace" (4:16). Grace is basically unmerited favor, and looking back (considering that we screwed up), it definitely fits what God did for us. None of us deserve God's righteousness -- but Jesus gave it anyway with the "great exchange" (taking on our sin and providing his righteousness).
And once again, the grace offer is not only for those under the Mosaic Law but "also to those who are of the faith of Abraham" (4:16). So the grace thing is meant to be widespread and available for anyone who believes in Jesus. In particular, Abraham believed God, who was the one "who gives life to the dead and calls those things which do not exist as though they did" (4:17).
We see both these things through God's righteousness plan. God's power is how Jesus rose from the dead, and God is also basically creator who created said righteousness plan in the first place. He provided the way for us to receive God's righteousness (via Jesus and what he did on behalf of us) when we sadly couldn't keep up with God's standards. This is the bulk of the "gospel" (good news) where anyone (even YOU) can receive by believing in Jesus as Lord and Savior.