Fourth stanza

Once out of nature I shall never take
My bodily form from any natural thing,
But such a form as Grecian goldsmiths make
Of hammered gold and gold enamelling
To keep a drowsy Emperor awake;
Or set upon a golden bough to sing
To lords and ladies of Byzantium
Of what is past, or passing, or to come.

Stanza four continues the speaker’s prayer. Once transformed by the gyre of fate, the speaker vows never to return to not only his body but “any natural thing” as well (IV.26). He instead would choose to be a gold mechanical bird, singing to the emperors, lords, and ladies of Byzantium of “what is past, or passing, or to come” (IV.32).

Once again, “the form as Grecian goldsmiths make” (IV.27) is bird imagery, alluding to the Byzantine Emperor Theophilos who had made for himself mechanical golden birds that sang upon the branches of a golden tree. Taken literally, the speaker’s goal is to become a mechanical bird capable of entertaining the ears of mortals with the song of time itself. This eternal form may seem more like a curse than a worthy life pursuit since the speaker’s soul now resembles something metallic and inhuman. However, this singing metal bird connects with the singing natural birds in the first stanza. The speaker’s desired form may be interpreted as a balanced synthesis of the natural world and the eternal world. The speaker could sing his soul without the constraints of time, singing of past, present, and future. The synthetic quality of his form may also represent the works of art left for future humanity. The art of the goldsmiths (IV.27), the mosaic (III.18), and the monuments (I.8, II.14) all have an eternal quality. Either by art or by spiritual ascension, the speaker would defy the death mandated by the natural world (Britannica).

See 'The Golden Bough' by Sir James Frazer.