G5 is >G4 because it can operate at levels beyond G4, not because the weakest of G5 Luffy's attacks are stronger than all of the G4 attacks.
a G2 punch is stronger than a base punch, a G3 punch is stronger than a G2 punch, a G4 punch is stronger than a G3 punch.
G5 being G5 necessitates it’s punches are stronger than G4.