ultimate = last
penultimate = next to the last
antepenultimate = last but two; before the next to the last; third from the last(?)
preantepenultimate = last but three; fourth from the last(?)

the ultimate definition for the last two words seems wrong to me, and in fact is contradicted by some sources. what does our usage panel say?

..(I'd say, 'fourth from the end' for preantepenultimate.)