Kinemon and Kanjuro: they know about Oden and how incredible he was power level wise
Also Kinemon and Kanjuro after witnessing Zoro's prowess:
I'm not saying that Zoro was already equal to Oden, but he managed to impress 2 of Oden's samurai, who are well used to know the power of a so called Top Tier.
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The old man Chinjao fought against pre G4 Luffy and is wondering himself that a man with such power is only an underling, because here Zoro showed to be stronger than pre g4 Luffy and hence the Chinjao's wondering, otherwise he wouldn't have questioned himself.
He couldn't believe that an underling could be stronger than the boss.
That was when nobody knew about G4 though, he didn't know about Luffy's powerup at that moment so he was believing that Zoro was stronger than Luffy.
Side Note: If Luffy has G4 then Zoro has Ashura, although I believe Luffy G4 was stronger than Ashura Zoro in Dressrosa...but still, they were close.
For me:
Luffy G4 > Zoro Ashura > Zoro > Luffy G2/3 > Luffy