In the fourth stanza, the speaker imagines the nearby town emptying as its population comes to its leafy outskirts to celebrate the religious festival depicted on the urn. What the speaker is envisioning, the empty town, is offstage from what is shown on the urn. The speaker emphasizes that this town is indeterminate: it could be by a river or a seashore or perhaps on a mountain top; the reader cannot know, as they cannot see it. However, wherever it may be, the town is empty this particular morning because of the festival.

The speaker ends the stanza by stating that, wherever it is, there is

not a soul to tell
Why thou art desolate, can e'er return.

In other words, just as the figures on the urn are frozen in a moment in time, so, too, is the town from which they come. It will be forever mysteriously "desolate" or empty because the people at the festival can't ever return to explain why it is empty.

This line emphasizes the eternal or static nature of art. Because the scene at the festival is occurring on an urn, it will not change as human life does. The eternal moment of bliss depicted means an eternal moment of desolation or emptiness somewhere else.

At this point, the speaker is about to transition out of his intense feelings of identification with the urn so that he can once again observe it from afar. His imagination wandering away from the scene itself to think about the town is the beginning of that process.