I think it's clear that the last round before the 4th fire is a full round and income is collected. But, what about the rare case when the 3rd fire is the penultimate card? Does that single card count as a round?
I'd say no as it would add an extra round to the game. Just carry out fires 3 and 4 consecutively with no income for either.
This does not happen. The fire does not enter the board, the last one enters.
I'm a little confused as to your meaning.
Attention: Occasionally it may happen that the card 3rd fire (D) is the final autumn card and sits right above the card 4th fire (E). In that event, the round ends after revealing and carrying out the 3rd fire, but carry out the 4th fire immediately.
After that, the game ends.
Clearly both the 3rd and 4th fires occur and are scored. The only questionable thing is does the 3rd fire count as an entire round with income? It refers to it as a round but says the round ends immediately once the third fire is resolved. That's why I say no income.
Remember, if the 3rd fire is the last card in the deck before the 4th fire it will always be drawn on it's own. It won't appear as a part of a round with other cards. The game makes you remove cards at all player counts just so it will work out this way.