Luffy has probably gotten stronger after his battle with Doflamingo. The whole speech about Luffy not running away from an Admiral and proclaiming he will blast all the Yonko and Admirals away is completely useless if he can only give Jozu mid diff.Not really relevant. If Luffy couldn't kill an already severely injured Doffy with G4 then someone who is implicatively more durable then Doffy at full health should pretty much just win because Luffy is too weak. Doesn't matter what Luffy does, Jozu can literally do nothing and win. Why would anyone put this any diff about mid diff given this?
Offensively G4 craps on any attack Jozu has shown, if Luffy can give an on guard admiral a bruise with one of his lesser G3 attacks, imagine the damage he can do with the stronger attacks and G4.