Intelligent Systems originally planned to do a second designer's notes page for FE4, but never did it in the end. The first page's contents were included in the Fire Emblem: Treasure book, while it's likely the second page's contents were included in a different book (which I've been hoping to get ahold of). The subjects listed for the other book include Akaneia and Judgral's relationship and the Curse of the Gaebolg (the latter of which I might cover later).
Akaneia and Jugdral's relationship
According to this Japanese fansite:
"The continent of Akaneia and the continent of Jugdral co-exist in the same world. However the difference in level of civilisation between the two is as different as heaven and earth. The Miracle of Darna of Judgral was the battle between the Divine Dragons and Medeus's Earth Dragons, which was known as the Guardian God Naga's Battle."
Meanwhile, according to the Japanese wikipedia page:
"During Akaneia year -740 (1340 years before the beginning of FE1), the Guardian God Naga's Battle, Archbishop Galle witnessed the Dark God's descent at Jugdral (which occured in Jugdral year 440, around 320 years before the beginning of FE4). So there is a 1000 year gap between FE1 and FE4 (with FE4 taking place first chronologically)."