There are some fascinating hints at the structure of the New Testament.
4 - Gospels --> Acts (1 book) --> 7 cities/churches
4 - Letters to People --> Hebrews (1 book) --> 7 - Letters from the friends and family of Jesus
In the last seven, first and last are relatives of Jesus, middle are best friends of Jesus and apostles.
Regarding the 7 books after Acts, the central city is Ephesus which is also the central city in the letters to the seven churches in Revelation 2-3.
I have no idea how to put this together, but I think it's a step in the right direction