In the fourth stanza, the speaker considers the "wild" or daring men who chase the sun's journey. The sun begins each day with a gorgeous sunrise, which is often representative of new life in literature. A sunrise holds hope and promise, flooding the sky with gorgeous colors. New life, likewise, is promising and beautiful, and it is easy to hold on to these ideals as the sun begins its journey across the sky.
Daring men, then, celebrate the beautiful journey of life even as it becomes clear that sunset is on the horizon. As the sun begins to dip in the western sky, it again becomes fantastically stunning, captivating onlookers with its majestic display. This one final burst of color is an indicator that the day—or life, in this poem—is nearing its end.
Yet the most daring of men continue to "sing" through this flight of the sun. They are so busy celebrating the splendor of sunset that they don't realize the truth of that final display of color until it is "too late." Their celebratory actions actually steer them toward death, and they fail to recognize that the end of life is imminent, because they are so absorbed in living.
Nevertheless, these daring souls refuse to slip quietly from the earth, instead raging against the "dying of the light."