The form of a Whitman poem can be tough to judge. First of all, Whitman poems typically have really long lines, which often run over into the next line because they're longer than our pages are wide.
"Pioneers! O Pioneers!" is no exception. We found versions of this poem with stanzas that look to be between 3 and 6 lines. But no two look alike. Okay, they do, but seriously, there are tons of different ways this poem can play out on the page.
We think we've chosen correctly to present it in four line stanzas (but we're not guaranteeing that's the only right way to do it). But for the sake of clarity, we're saying that the poem is in un-rhymed, un-metered quatrains (four line stanzas).
It's also worth noting that the refrain—the repeating last line "Pioneers! O Pioneers!"—is definitely a part of the form. Looking closer, it's also hard not to notice that the first line of the stanzas tends to hover around seven or eight syllables, and is always shorter than the second and third lines. Those lines are almost always longer, and build up momentum that culminates in the refrain.
And that refrain acts kind of like an exclamation point at the end of each stanza. So, although some stanzas carry on where the last left off, the way that refrain punctuates each one allows the poem to start up a whole new thought or topic with each stanza, if it wants to.