Whether the fourth act of Prometheus Unbound is grand finale or sublime afterthought, its internal character suggests that what urged Shelley toward its composition in the spring of 1819 was the desire to produce an appropriately magnificent celebration for all the blessings attendant on the union of Prometheus and Asia. As befits their standing as the parents and originals of all this “boundless, overflowing bursting gladness” (320), neither Asia nor Prometheus plays any role in this ceremony of praise. This play within the play is performed not by the two principals of Prometheus Unbound, but for them and their regenerative powers.

The act's opening stage direction names “a Part of the Forest near the Cave of Prometheus” as the scene for a series of performances consisting of: an introductory ensemble of choral song and dance performed by the paired “spirits of the human mind” and the new hours or seasons of human history; a masque-like pageant of Earth and Moon, coming to extended vocal life in the love duet between tenor Earth and soprano Moon; and a final valedictory from Demogorgon.