Who are the Yonkou before the current Yonkou..?

I'm just thinking of this topic..
Who are the Yonkou before the current Yonkou..
We knew about FOUR Yonkou..
i) WhiteBeard
ii) Kaido
iii) Big Mom
iv) Shanks

But could it be that before them..
There are Yonkou that Oda never mention to us yet..
Example: Shanks become Yonkou.. So who would be the Yonkou that he defeated..?
Yeah..
We can't even speculate about this topic..
Because this is far beyond our concern..
Because the answers left to Oda himself..
Maybe there's no point to him to mentioned about it..
Or maybe the timing is not quite good for him to tell us about the ex-Yonkou..

But certain thing does..
Like in MarineFord arc..
Oda stated that after WhiteBeard been killed by BlackBeard..
BlackBeard gain few islands under WhiteBeard and he become the next Yonkou..
After BlackBeard, many supernovas planned to defeat Yonkou so they would become Yonkou..
Example: Captain Eustass Kid & Killer makes ally with Basil Hawkin & Scratchmen Apoo.. They planned to defeat Shanks the Red Hair..

Did Roger was a Yonkou before he became Pirate King..?
If that so..
Who could be the ex-Yonkou that he defeated..?
Shiki The Lion..?

Why am I asking about this..?
Because maybe in WCI..
We could see the similar thing happens..
As Big Mom grab this Yonkou's title from someone she knew (maybe Mother Caramel)..
Her Yonkou's title will be grab by someone that she knew also (maybe Pudding)..
I so starving right now because I'm thinking about Caramel Pudding..
Is this a hint from Oda..?

What do you guys think of this topic..?
Please share your thought with me.. ^^