The Four Emperors are some of the most powerful and influential pirates in all of One Piece. Not only is this due to their various Devil Fruit powers, but also because they have proven themselves to be capable of forming alliances amongst their crews and the crews of others, whether that's by threatening them like Big Mom or through sheer respect like Whitebeard. With this in mind, it's easy to see why Luffy is being hailed as the unofficial "Fifth Emperor" by the in-universe media. Not only has Luffy proven himself a formidable opponent numerous times, but he's also been able to prove that he can bring others together via the Straw Hat Grand Fleet.
The Four Emperors make up one part of the Three Great Powers that have influence in the world of One Piece, with the other two being the Marines and the Special Science Group. They are some of the most influential people that reside within the New World, and while they aren't allies with each other, they aren't enemies either. They all rule their respective territories separately from each other. The current Emperors are Shanks, Big Mom, Blackbeard and Kaido.
Whitebeard was one of the original Emperors and also served with Big Mom and Kaido as members of the Rocks Pirates. However, after he was killed by Blackbeard, his title went to his killer. All four of the Emperors have the ability to use Haki in some fashion and Shanks is the only member to not have Devil Fruit powers. He's also the only member to be of relatively normal human size, too.
It isn't only the Devil Fruit and Haki that make the Emperors powerful, though. Each captain commands large fleets, with Big Mom having the largest simply because of her many children. Because the majority of them are loyal to her, she's been able to bring other crews into her own and spread her influence farther than other captains. Marriages also play a major part in garnering more crew members, as having a child marry the captain of another crew would in turn create an alliance between Big Mom's crew and the other.
Not only this, but each Emperor holds many islands and areas within their territories. Big Mom holds influence over Fish Man Island after taking it into her territory after the death of Whitebeard, so fishmen in other crews would hesitate to go against her for fear of her harming their home. We've also seen how she conspires with the governments and influential families in order to get what she wants.
Influence is just as powerful as fighting abilities when it comes to politics. It also breeds distrust among the leaders, which is how these powers balance each other. Nobody wants to step on the toes of the other leaders for fear of retaliation by someone with a larger fleet, or worse, an alliance with another Emperor. This is what also keeps the balance between the Three Great Powers.
One can conclude that the title of "Emperor" is one gained either through inheritance or usurpation, as was the case with Blackbeard. Should Luffy defeat Kaido, it's a safe assumption that he would receive Kaido's title as an Emperor upon his victory. However, his defeat could also have the opposite effect, and the entire Emperor group could crumble because of Luffy's actions against Big Mom during the Whole Cake Island Arc and the Wano Arc. There is the possibility that the upcoming chapters could resolve this issue by having Big Mom go down before Kaido, or even have Kaido defeat Big Mom before fighting Luffy. There's still a lot that can happen. We'll just have to wait and see how things play out from here to see if Luffy will claim the title of Emperor officially.
