Both Shanks and BB have 4B bounty.
And if we really following the parallel , Kizaru will win , which very unlikely to happen.
Kizaru and Luffy are both worth 3 billion. That's the set up.
"3 billion Law lost Shanks.
3 billion Kidd lost to Blackbeard.
3 billion Garp lost to Kuzan.
Now we have 3 billion Luffy vs 3 billion Kizaru. Who will prevail??"
It's obvious as hell