Stanza IV Summary

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

Who are these coming to the sacrifice?

Just when we thought the speaker might faint from the steamy, sticky atmosphere of the lovers, he manages to turn his attention to other things.
Now the speaker is looking at the third scene on the urn, which depicts an animal sacrifice.
Just as in stanza I, the speaking is leaning in and trying to figure out what is going on in the scene. In stanza I he asked "What," and now he asks, "Who?" There seem to be people coming to watch the sacrifice.

To what green altar, O mysterious priest,
Lead'st thou that heifer lowing at the skies,
And all her silken flanks with garlands drest?

Now our speaker talks to the priest on the urn, asking him, "Hey, where are you headed?"
He wants to know to "what green altar" he is taking a cow ("heifer").
In classical times, an altar was a place where sacrifices were carried out, and this one is covered with leaves and vegetation that make it green. The poor cow must know what’s coming, because it moans or "lows" at the sky.
Its sides ("flanks") are dressed in a string or "garland" of flowers. This cow is a holy object, destined for the gods.

What little town by river or sea shore,
Or mountain-built with peaceful citadel,
Is emptied of this folk, this pious morn?

We can now piece together the whole third scene. There’s a priest, a cow, a green altar, and a crowd of people following behind in anticipation of the sacrifice.
The speaker infers that this crowd must have come from somewhere, from some "little town," but the town isn’t depicted, so he has to imagine what it must look like.
He imagines things in the world of the urn just like we, the readers, imagine what is going on in the poem. Hm. Very curious.
This scene doesn’t have anything besides people and cows in it, but he comes up with a few guesses as to what the town looks like. It is either a.) by a river, b.) by a sea-shore, or c.) on a mountain.
If it’s on a mountain, he imagines a small fortress called a "citadel" must protect it. But there isn’t a great need to be defended, so the citadel is "peaceful."
This truly is a perfect world. Everyone is outside, enjoying the weather and looking forward to the ritual. The town is "emptied" because it is a "pious" or holy morning.
(The Ancient Greeks were pagans who believed in a lot of different, human-like gods, representing natural events like the sun and the seasons.)

And, little town, thy streets for evermore
Will silent be; and not a soul to tell
Why thou art desolate, can e'er return.

The speaker talks to the town to inform it that its streets will always be "silent" and "desolate" of people.
Although the speaker knows that everyone is headed to a sacrifice, he doesn’t know what the sacrifice is for, and he can never find out because there is "not a soul, to tell" the reason for the holy day.