The Elite Four are the four strongest trainers of a region's Pokémon League, fought consecutively in a single gauntlet with no Pokémon Center healing between battles except the items you carry. Defeating all four earns a battle against the Champion for the title.
The Elite Four have closed out the main story since Pokémon Red and Blue. You enter the League, heal once, and then the door locks behind the run: four battles in a row, each trainer themed by type or strategy, and your only recovery is the Potions and Revives in your bag. In Generations I to IV the order was fixed, while from Generation V onward most games let you choose the order of the four rooms, which lets you plan around your weakest matchup.
The format tests a different skill than Gym battles. A Gym asks whether your team can beat one type today. The Elite Four ask whether your team and your item stock survive four hard battles in sequence, because HP and PP spent in the first battle are still gone in the fourth. Players plan leads, PP conservation, and which Pokémon can be allowed to faint and be revived later.
Members also recur across games as characters, and rematch teams after the story are stronger, with new Pokémon and held items. Alola shook up the format in Generation VII by founding its League during the story and making the player its first Champion, and Paldea's Elite Four in Generation IX are fought at the League after an interview and an exam style application.
In play"Pack Full Restores for the Elite Four, because there is no healing between those fights."