Here is a small theory about the history of Yonko and a quick answer to the question : Why only 4 pirate emperors ?
The manga tells us that the great era of pirates starts after Roger's death. Before him, there are only few pirates, so I don't think Yonko existed before and during Roger's era.
The pirate king is the master of all pirates. So I guess Yonko and Pirate King can not existed in the same time. What will be the authority of a Yonko compared to a Pirate King ? Nothing at all.
We know Roger has gathered all 4 Road Poneglyphe to discover Rough Tell and to be the Pirate King. What I think is that after Roger, the 4 Road Poneglyphe has been shared between 4 powerful pirates. Having a Road Poneglyphe was not enough to become the new pirate king, but enough to self-proclamed you as Pirate Emperor and have some territory into your authority.
This part of the theory is just about the creation of the Yonko but not about the actual meaning of being a Yonko (which have been changed over the time). By the way, it's not important to know who were the Yonko 26 year ago to agree this first part of the theory.
The second part of the theory is about the missing Road Poneglyphe. Imagine than the Yonko just after Roger's death were Kaido, White Beard, Big Mom and Shanks. We know than Kaido and Big Mom still have a Road Poneglyphe. The Road Poneglyphe of Shanks should be the one of Zou because Shanks and the Minks are friends. So the question could be "where is the Road Poneglyphe of White Beard ?". I don't have real suggestions to do. What do you think about this ?
The manga tells us that the great era of pirates starts after Roger's death. Before him, there are only few pirates, so I don't think Yonko existed before and during Roger's era.
The pirate king is the master of all pirates. So I guess Yonko and Pirate King can not existed in the same time. What will be the authority of a Yonko compared to a Pirate King ? Nothing at all.
We know Roger has gathered all 4 Road Poneglyphe to discover Rough Tell and to be the Pirate King. What I think is that after Roger, the 4 Road Poneglyphe has been shared between 4 powerful pirates. Having a Road Poneglyphe was not enough to become the new pirate king, but enough to self-proclamed you as Pirate Emperor and have some territory into your authority.
This part of the theory is just about the creation of the Yonko but not about the actual meaning of being a Yonko (which have been changed over the time). By the way, it's not important to know who were the Yonko 26 year ago to agree this first part of the theory.
The second part of the theory is about the missing Road Poneglyphe. Imagine than the Yonko just after Roger's death were Kaido, White Beard, Big Mom and Shanks. We know than Kaido and Big Mom still have a Road Poneglyphe. The Road Poneglyphe of Shanks should be the one of Zou because Shanks and the Minks are friends. So the question could be "where is the Road Poneglyphe of White Beard ?". I don't have real suggestions to do. What do you think about this ?