One Piece novel A is an official two-part novel series about Portgas D. Ace’s journey as a pirate.
The novel provides a lot of important information about the Yonko:
1- Roger, Whitebeard, Shiki and Big Mom were the rulers of the New world in the previous Era and only Big Mom remains from that Era. Kaido and Shanks are from the generation after that.
2- Kaido is said to be physically stronger than old Whitebeard but Whitebeard is great at piracy and a far superior pirate than Kaido.
3- Thatch says to become a Yonko, one must be at least as strong as Shanks implying Shanks is the weakest at that time (no surprise cause he was the youngest Yonko).
4- Shanks had several duels with Mihawk and every one of them have made the headlines. Furthermore, most of Shanks’ fame comes from those duels. It is also said that he does not seek to become the Pirate King.
5- Blackbeard gave Shanks the scar just before Shanks went to the East Blue and met Luffy. It is reiterated that Teach is a beastly and ruthless fighter.