Remember when we see Luffy firsttime in action against a Pacifista. Oda draw him via he casually using G2 drawback and CoA to one shot one Pacifista.
I think Luffy using G4 now for these ships is similar, Oda telling us which these panels that Luffy increase again, he using casually G4 Boundman mean these points:
>His CoA power and Haki control is far greater now, he increase his CoA a lot
>He can now use casual G4Boundman without having a big drawback, also he probably can use G4 far longer and better the before
>G4 showing in this way probably mean that he having even a far greater new form/skill which he probably use later as trumpfcard against Kaido ala G5
Oda showcase Luffy in a different lvl, we see that he increase a lot, his old version(Fishman arc Luffy&Dressrosa Luffy)are nothing to compare them to his new beast form.
Like that a lot and that means also Oda saving a insand new skill for Luffy Later against Kaido.