Luffy will have to catch up to him or even surpass him in One Piece. But, when it comes to exhaustion and shortening of lifespan as a side effect, the Gear that was more similar to Gear 4 was Gear 2.

Luffy has gained additional beneficial side effects beyond just the Gear 5 transformation. Luffy is first seen partially transforming into Gear 4 in One Piece Chapter #1127. The nature of this ...