Why doesn't Luffy combine Gear 2/3?

[Diamond Ryan]

With his new mastery over the gears, shouldn't he be able to do it without nearly as much strain on his body as it did in Thriller Bark?

 

[Yoshi]

Probably because Gear 4 is just that much stronger.

 

[sei'taer]

Gear 4 seems faster than gear 2, and more powerful than gear 3.

 

[Captain Cadaver]

That, and the King Kong Gun seems to already stack Gear 3rd on top of Gear 4th. Whilst it is far more taxing than his Jet Shell in Thriller Bark, going Gear 4th to fight an opponent still seems more efficient than stacking Gears for one attack when Luffy can still handle G4 for 30 minutes.