Just when Big Mom called Luffy coward/spineless, at that moment Luffy used G4, but it was quite a brief moment to my guess for seconds. It's impossible for Katakuri to notice it during all the commotion.Happen to remember the chapter I remember him punching her hand but I don't remember his body being in G4 when he did I seem to remember it being a G3 punch but we may be thinking of different moments.
This is the scene I remember
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Nevermind I found it