User blog comment:FairyRaven/Fairy Tail Chapter 401: Igneel vs. Acnologia, Review and Commentary/@comment-77.194.131.12-20140922125827/@comment-77.194.131.12-20140922130913

i reach that conclusion following Igneel's appearance and speech :

Why Igneel is the only one who appeared and not the others, because they must be sealed by something, and I think Igneel's seal broke because of END's book, so 2 other dragons are missing, so if END as a flame dragon can break Igneel's seal it must be have 2 other books with the same powers.

For the last one, I think there must have something related to Layla and the gold keys, because of the CSK's speech.

3+1 = 4 and Mashima put the word END every where, so 4 + END = the four horsemen of the Apocalypse