@Ordinary:
If Oda wanted to make a Yonko cover, I'm sure there would be Big Mom or Kaido in it too.
At the time volume 45 was published, WB and Shanks have been the only confirmed Yonkou so far, please pay attention.
Maybe Kaidou was mentioned by Moria one time in vol 46 or 47, but there was only speculation that Kaidou was a Yonkou back then.
Oda only wanted to display that BB is on same lvl as WB and Shanks, which means Yonkou lvl.
Look at vol 45 cover: WB's face is drawn from the side as well as Shanks' and BB's. Ace is drawn completely different. You see his whole upper Body and halfway from the front. When I saw the cover I was absolutely sure that BB would defeat Ace and become Yonkou one day. Back then I got lol'd at for this speculation. People were like (imagine stupid voice here):"BB can't be a Yonkou! He is already a Chichibukai!" (stupid voice over)
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@Hari:
There was no haki during Skypeia. It was just retconed to be one later.
Kenbunshoku was introduced during Amazon Lily and there wasn't any mention of any hearing attributes to it since, only visual/sensual.
In the chapter right b4 the 2yrs-TS (and the long 4-weeks break of Oda) Rayleigh taught Luffy the 3 "colours" of haki. He said that the Color of Observation (CoO) is called Mantra in Skypia. Coby's has awakened the same way as it seems like Usoppe's: all of a sudden he heard voices in his head, just like Aisa did during the Skypian "tournament".