Luffy using G4 against Number is kind of underwhelming imo....He didn't use G4 initially against Ulti ( who he considered as strong).... He didn't go G4 against BM ( he didn't want to fight her now tbf).....
He now can use advanced CoA....It would have been good if Oda showed us Luffy defeating Numbers with G2/G3 plus advanced CoA imo.....