Until recently, Luffy never used Gear 4 easily, he always used it as a last resort, a trump card, he saved it up for the worst case scenario and only used it after he has tried out all of his other move sets (base form, Gear 2, Gear 3), but all of that changed.
After his training in Udon, Luffy started to use Gear 4 more casually without any seen repercussions. This is drastically different than what Gear 4 usage used to be, and regardless in “small bursts” Luffy is using it on enemies that can easily be taken down with Gear 3.
Luffy even used it on a ship that can be taken down with pre timeskip Gear 3 without Haki. Putting all that together, I guess Oda is trying to put emphasis on the fact that Luffy has now “mastered” Gear 4. So Gear 4 is no more the energy draining trump card/last resort that used to be prior to Luffy’s training in Udon.
Gear 4 can now casually be used like Gear 2/3. We also see Luffy using Haki after using Gear 4 in chapter 991, when he sues future sight not that long after he uses Gear 4.
This means that there’s probably a new huge trump card that Luffy is saving up (Gear 5?).
*Theory by BigassMF45
