If you're interested in watching Luffy's Gear 4 debut, I recommend starting from Episode 699 to get the full context of the battle against Doflamingo.
Debuts in Episode 806 during the fight against Charlotte Cracker. Luffy used this "Full Stomach" version of Tankman after consuming massive amounts of Cracker’s biscuits to boost his defensive durability.
However, the sequence of events leading to Gear 4's debut starts a bit earlier: