Looking at the enemies luffy is about to face in future( Kaido, Akainu, Blackbeard and World Government etc.) I don't think Gear 4 is powerful enough.
We have seen three form of Gear 4 so far Boundman, Tankman and Snakeman. He used all three to fight Yonko commander level characters.
There are much stronger characters out there that Luffy needs to fight.
Is Gear 5 needed in coming time?
What do you think Gear 5 will be based on?
Does luffy already know how to use it or he will learn sometime in future?
Will we see it in Wano?
I really liked these designs. He is somewhat looking like a Devil.