We’ve seen the Yonkos doing some awesome things in the story but we haven’t witnessed their full powers and capabilities so I believe each of the Yonko specializes on one specific Haki type:
Busoshoku Haki “Armament Haki”
Busoshoku Haki is a form of Haki that allows the user to use their spirit to create, in essence, an invisible armor around themselves. Notably, this form of Haki can be used to injure Logia Devil Fruit users. Now 2 of the Yonko specializes in this.
Now it’s said that it’s impossible to hurt him so Kaido’s body is fully covered with Busoshoku Haki to such a high level that it’s impossible to break it.
This is a high possibility since he is not connected to any devil fruits at the moment and all the Yonkos have the ability of using haki at a really high level so I believe that Kaido would specialize in “Armament Haki”.
Big Mom has supreme mastery of Busoshoku Haki. By hardening her arm, she easily blocked Luffy’s Gear Fourth Kong Gun, a tremendous feat as Luffy’s Kong Gun is powerful enough to break through powerful Haki based defenses of highly skilled Busoshoku Haki users such as Doflamingo, Cracker, and Katakuri.
So Big Mom also has unscratchable body but when she was in a psychological crisis her unscratchable skin seemed to have lost its hardness.
So Big Mom’s spiritual armor seems to be lost when she’s in distress meaning that it’s weaker than Kaido’s Armament Haki.
Haoshoku Haki “Conqueror’s Haki”
Haoshoku Haki is a rare form of Haki that cannot be attained through training. Only one in several million people have this ability. This type of Haki allows the user to exert their willpower over others.
When he walked towards Whitebeard half the people on that ship fainted and he did this without directly looking at them while Big Mom had to scream to use her Haoshoku Haki.
Shanks could do this in his sleep.
Meaning that his Haoshoku Haki is superior to any character who could use this, also Rayleigh has to directly look at something to make his Haoshoku Haki work.
He could possibly use this on a admiral since Whitebeard called it a weird aura meaning that it’s different from his Haoshoku Haki.
Oda revealed that Shanks could KO 100,000 people at once.
Kenbunshoku Haki “Observation Haki”
Kenbunshoku Haki is a form of Haki that allows the user to sense the presence of others, even if they are concealed from view or too far to see naturally.
The last Yonko remaining is Blackbeard and the reason why Blackbeard specializes in this is because of his past battle with Shanks and his actions so far:
Marshall D. Teach
Ok now Blackbeard has a special Kenbunshoku Haki that allowed him to predict his opponents movements and some events in the near future:
• Shanks mentioned that he didn’t let his guard down when he fought Blackbeard meaning that he was able to avoid Shanks’ attacks and use that moment to give the scar on his eyes;
• Appearing at the right moment he was able to make Whitebeard run out of energy so he’d die – this is kind of a long shot but I think he was able to predict the time that Whitebeard was gonna die so he could arrive at the right moment;
• He went head to head with Ace – I think he knew he was going to win because he was cocky during the fight even though it was a draw he could have predicted the whole thing.
*Theory by King of beasts kaido