I concluded my analysis of the third stanza by crying "uncle" when I reached the last three lines. The main reason? I couldn't parse the grammar and didn't want to look the fool. Not that I need to parse it to understand what Keats says there—only that when you break out the spelunking gear and start exploring, things don't look like the maps say they should.
This tunnel here should delve into the deeps but shoots "far above" instead (28). That one there should plunge you into icy waters but leaves you with "a burning forehead, and a parching tongue" (30). You can hear the echo of "your leaves" (22) in "that leaves" (29) but your once "mad" echolocation" skillz" have diminished with disuse. So complain about those last three lines until you whine your parched throat raw, it will do you no good. If only you had friends ...
The fourth stanza, however, I can handle. It opens with that trope I should've christened something clever already. Sadly, "the trope of evocative ignorance" tops my shortlist. But I've got my best people working 'round the clock, and they're bound to come up with some witty thing sooner or later. Better sooner—but I digress.
I need to stop naming stuff and start describing it. Only, what is there to say about those opening lines that I haven't said about its predecessors? Quite a bit, actually, but in keeping with the idea that readings should be coherent, I'm disinclined to mention them. I still may, but I want to focus on ignorance.
The speaker, you'll remember, has no idea who the people depicted on the urn are—yet in the fourth stanza, he describes the coming sacrifice in such a way as to betray either insincerity or stunning unawareness. "What men or gods are these?" (8). Never mind! No longer an issue! These twits ain't gods. They're men.
On the one hand, the speaker's supposition seems sound—sacrifices are made to gods, not by them. On the other, why is that necessarily the case? I can imagine a polytheistic religion in which lesser gods sacrifice specimens from their lowing, mewling or mooing flocks to appease their superiors.
The speaker doesn't even consider this possibility, as he uses this portion of the frieze to jump from what is represented to what is entailed by that representation. This imaginative induction moves the "Ode" outside the tradition of ekphrasis and into a speculative mode empowered by the speaker's ignorance. In the first three stanzas, the speaker engaged in crypto-speculation—his professions of ignorance notwithstanding, he appeared to be describing the events depicted on the urn, appeared to know what he was talking about. In the fourth stanza, he exploits this impression to describe events not depicted on the urn with the same "authority" with which he described the ones that were. The reader's trust is inertial here, based less on what's been said than the fact that something has.
Because when you think about this stanza, the inhabitants of a hypothetical town abandon their "homes" to participate in the sacrifice—which, since it is depicted on the urn, means that they have abandon their "homes" forever. No one will ever know why the hypothetical town "is emptied of its folk, this pious morn" (37). We can only ever know that it is and sympathize accordingly. With whom are sympathizing here?
An anthropomorphized hypothetical town. Why? We've been led to by a speaker whose ignorance has granted him fantastic manipulative powers. It papers over the missing floorboards so convincing that we praise it while falling, comically, story-by-story before terminating on the cold cement of the basement floor. Next to the dryers and the pile of dirty laundry the cat mistook for a litter box. Our last moments are suffused with pain and the stench of stale ammonia, and with our last labored breaths we curse that Keats fellow and his inspired complications ...