4th Haki (Spoilers)
I'm just going to give a brief explanation of this because some people are really taking the piss and I can't be bothered, me tienen hasta los cojones. So I'll explain it now and then leave a detailed version as a side note in the story.
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I created the 4th Haki to form as an explanation as to why characters like Gol D. Roger and Rocks D. Xebec were so far stronger than their peers (who on their own already look quite invincible, to begin with). Another example of this is Mihawk herself. In cannon the male version uses a technique that produces a green aura and to this day it hasn't been explained what it is.
In simple terms, the way to unlock the 4th Haki is to become aware of the 4th Wall (and consequently aware that different world has different laws and limitations).
Veyron awoke this Haki because he was aware from the beginning that this world was not his and from his point of view he could do a lot more things in One Piece than he could in his original world. His psychokinesis is related to the way he comprehends the physical laws and their limitations (which don't exist in One Piece).
Mihawk managed to unlock it by continuously analysing his behaviour, the way he devised techniques and the way he lived his life in One Piece overall, which mixed with her own perception of the sword and awoke her 'Sword Intent'.
Aside from Veyron and Mihawk, the only alive character I have confirmed so far that has it is Imu (for those who don't know who he is, I don't intend to spoil anything).
It is not a technique that can be learnt by training or obtained by birth and hence there won't be many characters in the story that have it. But there are many characters that can recognise it since they saw it being used by Roger, Xebec and perhaps any other characters that used it in the past and are now deceased like Joyboy. Ex: Rayleigh, Garp.
I have only one more character from the main plot that I know I will give it to as it will fit the story, but I won't spoil who it is. I leave that for you to guess.