Who were the previous "Emperors of the Sea" Just like the way there have been multiple Kage's over the centuries (in the Naruto-verse), have there also been yonko's for previous generations? What i'm trying to grasp is if the Yonko title started with the "Big 4" (Whitebeard, Kaido, Big Mom, shanks). If that were the case then Shanks would have been the youngest,and most likely the newest addition at that time.

Or Did Shanks replace a previous Yonko just like how 


  Blackbeard replaced Whitebeard?
I don't think previous Yonko have ever been mentioned in One Piece. It would actually be quite odd if there were always exactly 4 strong pirate crews. Especially since they have already added Luffy to the bunch so it's technically not even Yonko anymore.

The title probably originated after Roger died and a few of the pirate crews dominated everyone else. But there isn't much to cite from the manga as Oda hasn't really revealed anything about Shanks' rise to the top.