Stanza IV Summary

Get out the microscope, because we’re going through this poem line-by-line.

Once out of nature I shall never take
My bodily form from any natural thing,

Like we were saying just a few lines ago, nature fades. Seasons change. People die.
Art, however, lasts a long, long time.
As our speaker decides, the best way to preserve part of yourself is to lodge that part in "unnatural" things like art.
Once our speaker moves into a sort of speculative mood, his options seem a lot better than they were in his present. It’s almost like he’s daydreaming about his next life. ("Do I want to be a cricket? Nope. Maybe a Swiss army knife? Hmm…")
Right now nature’s got him locked in her claws; after death, however, he’ll be free to return as something else.

But such a form as Grecian goldsmiths make
Of hammered gold and gold enamelling
To keep a drowsy Emperor awake;

Once he’s imagined himself as an artwork, our speaker spends these lines clarifying exactly what sort of art he’d like to be. Like the sages, he’s thinking that he’s solid "hammered" gold.
In case you’ve missed it, he emphasizes the gold part once more for good measure. He won’t just be hammered gold. Nope. He’ll be hammered gold and "gold enamelling." That’s some serious bling.
Notice at this point that he’s alluding to the gold that went into the sages’ mosaic in stanza three. Now, however, our speaker seems to be imagining himself as a sculpture of some sort.
Hammering gold was a technique frequently used in crafting Byzantine jewelry and sculptures.
Then again, maybe our guy’s imagining himself in his own mosaic, instead. We’ll leave that particular choice up to you.
And here’s where his imagination takes over. Because art sticks around for such a long time, it can have many audiences. Think about the last museum you went to. How many people passed by one picture in the time you were there?
It’s a weird sort of popularity, but our speaker seems to want it. He imagines his audience as a "drowsy Emperor." Hey, if you’re daydreaming, you might as well reach for the stars.

Or set upon a golden bough to sing
To lords and ladies of Byzantium
Of what is past, or passing, or to come.

Moving deeper into a reverie about his future form, our speaker loops back to the first stanza of the poem.
Envisioning himself "set upon a golden bough," our speaker seems to suggest that he’d like to be a bird. Unlike the birds of nature, however, which fall in "dying generations," he’ll be a golden bird. Nature transforms into art over the course of the poem.
This time, though, the birds actually have someone to listen to their song. The "lords and ladies of Byzantium" will turn to the golden bird as a touchstone, something that allows them to connect the past, present, and future.
Because art lasts for all time, it can be relevant to all ages. That, friends, is immortality. It’s not a bad gig if you can get it.