I think the confusion creeps in when you tried to specify
> antepenultimate: second? from the last [e.a.]

The common phrase I think is based on an elision:
Penultimate is ‘second [compared] to last’ whilst antepenultimate is ‘third to last’, leaving preantepenultimate in ‘fourth to last’ position (or the ‘last excepting three’ as per OED).