If you are asking for the exact , here is the definitive answer. The Debut Episode Luffy first activates Gear Fourth in Episode 726.
For One Piece fans, the wait for Luffy’s next power-up is always agonizing. Following the immense toll of Gear Second and Gear Third, fans knew a new level was coming. That moment arrived during the climactic battle of the Dressrosa Arc .
The episode is titled "Gear Fourth! The Bouncy Man." It originally aired on July 19, 2015.